Unionfs: xattr copyup fixes

Unionfs: xattr copyup fixes

Rewrote xattr copyup code more cleanly; documented it better; eliminate one
possible leak in error path; and ignore another impossible copyup-time error
which caused fanout invariant violations under memory-pressure conditions.

Don't use vmalloc when allocating xattr buffers, as the VFS no longer does
so (just use kmalloc).  Eliminate unionfs_xattr_free which is now just plain
kfree.

Signed-off-by: Erez Zadok <ezk@cs.sunysb.edu>