fat: Replace fat_sync_inode() with sync_inode_metadata()

fat: Replace fat_sync_inode() with sync_inode_metadata()

Use generic sync_inode_metadata() instead of fat_sync_inode() for
persisting inode metadata changes for DIRSYNC inodes. This slightly
simplifies code and also addresses a theoretical race where
fat_sync_inode() could return before all metadata buffers associated
with the inode were properly written out when racing with fsync(2).

Signed-off-by: Jan Kara <jack@suse.cz>
Link: https://patch.msgid.link/20260727104923.3828017-39-jack@suse.cz
Signed-off-by: Christian Brauner (Amutable) <brauner@kernel.org>