generic/454: actually set attr value for llamapirate subtest

generic/454: actually set attr value for llamapirate subtest

Ted reported that this test fails on his setup, and I noticed that I
forgot to actually set a value for the xattr.  In theory filesystems
support zero-byte xattrs, but we might as well set and check the values
so that we can make sure nobody got confused.

The actual test failure comes from attr 2.4.47 refusing to set a
zero-legnth xattr, whereas 2.5 and newer will.  That was changed in the
attr commit 0550d2bc989d39 ("Properly set and report empty attribute
values") prior to 2.4.48:

https://git.savannah.nongnu.org/cgit/attr.git/commit/?id=0550d2bc989d390eb25f7004ee0fae2dbc693a0d

Cc: fstests@vger.kernel.org # v2024.10.28
Fixes: 9c3762ceafd430 ("misc: amend unicode confusing name tests to check for hidden tag characters")
Reported-and-tested-by: tytso@mit.edu
Signed-off-by: Darrick J. Wong <djwong@kernel.org>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Zorro Lang <zlang@kernel.org>