generic: test creating and removing symlink xattrs

generic: test creating and removing symlink xattrs

This began as a regression test for the issues identified in "xfs: allow
symlinks with short remote targets".  To summarize, the kernel XFS code
does not convert a remote symlink back to a shortform symlink after
deleting the attr fork.  Recent attempts to tighten validation have
flagged this incorrectly, so we need a regression test to focus on this
dusty corner of the codebase.

However, there's nothing in here that's xfs-specific so it's a generic
test.

Signed-off-by: Darrick J. Wong <djwong@kernel.org>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Zorro Lang <zlang@kernel.org>