ext2: create ext2_msg_fc for use during parsing

Rather than send a NULL sb to ext2_msg, which omits the s_id from
messages, create a new ext2_msg_fc which is able to provide this
information from the filesystem context *fc when parsing.

Signed-off-by: Eric Sandeen <sandeen@redhat.com>
Signed-off-by: Jan Kara <jack@suse.cz>
Link: https://patch.msgid.link/20250223201014.7541-3-sandeen@redhat.com