gfs2: gfs2_ail_empty_gl no log flush on error

Before this patch, function gfs2_ail_empty_gl called gfs2_log_flush even
in cases where it encountered an error. It should probably skip the log
flush and leave the file system in an inconsistent state, letting a
subsequent withdraw force the journal to be replayed to reestablish
metadata consistency.

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