Add migrations for SMB-related fields

In the DragonFish release we removed the clustering feature from
TrueNAS. In prior versions we had some SMB share presets that were
exposed only to clustering users via the webui, but were still
accessible via the API. These presets existed for TrueCommand purposes
to simplify cluster share creation. Unfortunately, some users may have
set these via our APIs on non-clustered shares, and so this commit adds
a migration to convert them to an appropriate SMB share configuration
post-Cobia.

Due to a longstanding bug in SCALE, there were circumstances in which
pam_mkhomedir would create the directory `/nonexistent` when users had
SMB homes shares. Starting in dragonfish / is readonly with the result
that the pam_session for users with a home directory of `/nonexistent`
to fail, resulting in SMB access being rejected in "obey pam
restrictions" is set. This commit adds a migration to force home
directories of SMB users to change to /var/empty if it was set to
/nonexistent.

(cherry picked from commit c268100)

src/middlewared/middlewared/alembic/versions/24.04/2024-05-01_15-55_smb_migrate_values.py

@@ -0,0 +1,63 @@
+"""Fix up SMB paramters and users
+
+Revision ID: f38c2bbe776a
+Revises: d774066c6c0c
+Create Date: 2024-05-01 15:55:42.754331+00:00
+
+"""
+from alembic import op
+import sqlalchemy as sa
+
+
+# revision identifiers, used by Alembic.
+revision = 'f38c2bbe776a'
+down_revision = 'd774066c6c0c'
+branch_labels = None
+depends_on = None
+
+SHARE_TABLE = "sharing_cifs_share"
+PURPOSE_KEY = "cifs_purpose"
+READONLY_KEY = "cifs_ro"
+
+USER_TABLE = "account_bsdusers"
+SMB_KEY = "bsdusr_smb"
+HOME_KEY = "bsdusr_home"
+LEGACY_HOME = "/nonexistent"
+EMPTY_DIR = "/var/empty"
+
+
+def upgrade():
+    conn = op.get_bind()
+
+    # convert any cluster READ_ONLY shares to a default share
+    # with readonly checked
+    stmnt = (
+        f"UPDATE {SHARE_TABLE} "
+        f"SET {PURPOSE_KEY} = ?, {READONLY_KEY} = ? "
+        f"WHERE {PURPOSE_KEY} = ?"
+    )
+    conn.execute(stmnt, ['DEFAULT_SHARE', 1, 'READ_ONLY'])
+
+    # convert any cluster DEFAULT_CLUSTER_SHARE shares to
+    # DEFAULT_SHARE
+    stmnt = (
+        f"UPDATE {SHARE_TABLE} "
+        f"SET {PURPOSE_KEY} = ? "
+        f"WHERE {PURPOSE_KEY} = ?"
+    )
+    conn.execute(stmnt, ['DEFAULT_SHARE', 'DEFAULT_CLUSTER_SHARE'])
+
+    # convert any SMB users with a home directory of `/nonexistent` to
+    # having a home directory of `/var/empty`
+    stmnt = (
+       f"UPDATE {USER_TABLE} "
+       f"SET {HOME_KEY} = ? "
+       f"WHERE {HOME_KEY} = ? AND {SMB_KEY} = ?"
+    )
+    conn.execute(stmnt, [EMPTY_DIR, LEGACY_HOME, 1])
+
+
+def downgrade():
+    # ### commands auto generated by Alembic - please adjust! ###
+    pass
+    # ### end Alembic commands ###