gfs2: Return more useful errors from gfs2_rgrp_send_discards()

The bug that 27ca8273 ("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>

fs/gfs2/rgrp.c

@@ -1315,7 +1315,7 @@ int gfs2_rgrp_send_discards(struct gfs2_sbd *sdp, u64 offset,
 	u64 blk;
 	sector_t start = 0;
 	sector_t nr_blks = 0;
-	int rv;
+	int rv = -EIO;
 	unsigned int x;
 	u32 trimmed = 0;
 	u8 diff;
@@ -1371,7 +1371,7 @@ int gfs2_rgrp_send_discards(struct gfs2_sbd *sdp, u64 offset,
 	if (sdp->sd_args.ar_discard)
 		fs_warn(sdp, "error %d on discard request, turning discards off for this filesystem\n", rv);
 	sdp->sd_args.ar_discard = 0;
-	return -EIO;
+	return rv;
 }
 
 /**