GFS2: Inline qd_trylock into gfs2_quota_unlock

The function qd_trylock was not a trylock despite its name and
can be inlined into gfs2_quota_unlock in order to make the
code a bit clearer. There should be no functional change as a
result of this patch.

Signed-off-by: Steven Whitehouse <swhiteho@redhat.com>
Cc: Abhijith Das <adas@redhat.com>