gfs2: Return more useful errors from gfs2_rgrp_send_discards()

gfs2: Return more useful errors from gfs2_rgrp_send_discards()

The bug that 895ee5709 ("gfs2: Make sure FITRIM minlen is rounded up to
fs block size") fixes was a little confusing as the user saw
"Input/output error" which masked the -EINVAL that sb_issue_discard()
returned.

sb_issue_discard() can fail for various reasons, so we should return its
return value from gfs2_rgrp_send_discards() to avoid all errors being
reported as IO errors.

This improves error reporting for FITRIM and makes no difference to the
-o discard code path because the return value from
gfs2_rgrp_send_discards() gets thrown away in that case (and the option
switches off). Presumably that's why it was ok to just return -EIO in
the past, before FITRIM was implemented.

Tested with xfstests.

Signed-off-by: Andrew Price <anprice@redhat.com>
Signed-off-by: Andreas Gruenbacher <agruenba@redhat.com>