gfs2: journal flush threshold fixes and cleanup

Commit f07b3520 ("GFS2: Made logd daemon take into account log
demand") changed gfs2_ail_flush_reqd() and gfs2_jrnl_flush_reqd() to
take sd_log_blks_needed into account, but the checks in
gfs2_log_commit() were not updated correspondingly.

Once that is fixed, gfs2_jrnl_flush_reqd() and gfs2_ail_flush_reqd() can
be used in gfs2_log_commit().  Make those two helpers available to
gfs2_log_commit() by defining them above gfs2_log_commit().

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