gfs2: Fix length of holes reported at end-of-file

gfs2: Fix length of holes reported at end-of-file

[ Upstream commit f3506eee81d1f700d9ee2d2f4a88fddb669ec032 ]

Fix the length of holes reported at the end of a file: the length is
relative to the beginning of the extent, not the seek position which is
rounded down to the filesystem block size.

This bug went unnoticed for some time, but is now caught by the
following assertion in iomap_iter_done():

  WARN_ON_ONCE(iter->iomap.offset + iter->iomap.length <= iter->pos)

Signed-off-by: Andreas Gruenbacher <agruenba@redhat.com>
Signed-off-by: Sasha Levin <sashal@kernel.org>