gfs2: Fix freeze consistency check in log_write_header

Functions gfs2_freeze_super() and gfs2_thaw_super() are using the
SDF_FROZEN flag to indicate when the filesystem is frozen, synchronized
by sd_freeze_mutex.  However, this doesn't prevent writes from happening
between the point of calling thaw_super() and the point where the
SDF_FROZEN flag is cleared, so the following assert can trigger in
log_write_header():

    gfs2_assert_withdraw(sdp, !test_bit(SDF_FROZEN, &sdp->sd_flags));

Fix that by checking for sb->s_writers.frozen != SB_FREEZE_COMPLETE in
log_write_header() instead.  To make sure that the filesystem-specific
part of freezing happens before sb->s_writers.frozen is set to
SB_FREEZE_COMPLETE, move that code from gfs2_freeze_locally() into
gfs2_freeze_fs() and hook that up to the .freeze_fs operation.

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