xfs: ignore discard return value

xfs: ignore discard return value

__blkdev_issue_discard() always returns 0, making all error checking
in XFS discard functions dead code.

Change xfs_discard_extents() return type to void, remove error variable,
error checking, and error logging for the __blkdev_issue_discard() call
in same function.

Update xfs_trim_perag_extents() and xfs_trim_rtgroup_extents() to
ignore the xfs_discard_extents() return value and error checking
code.

Update xfs_discard_rtdev_extents() to ignore __blkdev_issue_discard()
return value and error checking code.

Reviewed-by: Johannes Thumshirn <johannes.thumshirn@wdc.com>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Chaitanya Kulkarni <ckulkarnilinux@gmail.com>
Signed-off-by: Carlos Maiolino <cem@kernel.org>