gfs2: allow fallocate to max out quotas/fs efficiently

We can quickly get an estimate of how many blocks are available
for allocation restricted by quota and fs size respectively, using
the ap->allowed field in the gfs2_alloc_parms structure.
gfs2_quota_check() and gfs2_inplace_reserve() provide these values.

Once we have the total number of blocks available to us, we can
compute how many bytes of data can be written using those blocks
instead of guessing inefficiently.

Signed-off-by: Abhi Das <adas@redhat.com>
Signed-off-by: Bob Peterson <rpeterso@redhat.com>
Acked-by: Steven Whitehouse <swhiteho@redhat.com>