fsstress: fix attr_set naming

fsstress: fix attr_set naming

Back in 2020 I converted attr_set to lsetxattr, but neglected to notice
that the attr name now has to have the prefix "user." which attr_set
used to append for us.  Unfortunately nobody runs fsstress in verbose
mode so I didn't notice until now, and even then only because fuse2fs
stupidly accepts any name, even if that corrupts the filesystem.

Found by running generic/642 on fuse2fs.

Cc: fstests@vger.kernel.org # v2022.05.01
Fixes: 808f39a416c962 ("fsstress: stop using attr_set")
Signed-off-by: Darrick J. Wong <djwong@kernel.org>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Zorro Lang <zlang@kernel.org>