GFS2: Fix fstrim boundary conditions

GFS2: Fix fstrim boundary conditions

This patch correctly distinguishes two boundary conditions:

1. When the given range is entire within the unaccounted space between
   two rgrps, and
2. The range begins beyond the end of the filesystem

Also fix the unit of the returned value r.len (total trimming) to be in bytes
instead of the (incorrect) 512 byte blocks

With this patch, GFS2 passes multiple iterations of all the relevant xfstests
(251, 260, 288) with different fs block sizes.

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