jfs: Clean up xattr name mapping

Instead of stripping "os2." prefixes in __jfs_setxattr, make callers
strip them, as __jfs_getxattr already does.  With that change, use the
same name mapping function in jfs_{get,set,remove}xattr.

Signed-off-by: Andreas Gruenbacher <agruenba@redhat.com>
Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>