gfs2: Add support for IOMAP_ZERO

Add support for the IOMAP_ZERO iomap operation so that iomap_zero_range will
work as expected.  In the IOMAP_ZERO case, the caller of iomap_zero_range is
responsible for taking an exclusive glock on the inode, so we need no
additional locking in gfs2_iomap_begin.

Signed-off-by: Andreas Gruenbacher <agruenba@redhat.com>
Reviewed-by: Bob Peterson <rpeterso@redhat.com>