GFS2: Make two similar quota code fragments into a function

There should be no functional change bar the removal of a
test of the MS_READONLY flag which would never be reachable.
This merges the common code from qd_fish and qd_trylock into
a single function and calls it from both those places.

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