gfs2: bufdata allocation race

gfs2: bufdata allocation race

The locking in gfs2_trans_add_data() and gfs2_trans_add_meta() doesn't
follow the usual coding pattern of checking bh->b_private under lock,
allocating a new bufdata object with the locks dropped, and re-checking
once the lock has been reacquired.  Both functions set bh->b_private
without holding the buffer lock.  Fix that.

Also, in gfs2_trans_add_meta(), taking the folio lock during the
allocation doesn't actually do anything useful.

Signed-off-by: Andreas Gruenbacher <agruenba@redhat.com>