btrfs: optmize listxattr

The ->list handler is really not useful at all, because we always call
btrfs_xattr_generic_list anyway.  After this is done
find_btrfs_xattr_handler becomes unused, and it becomes obvious that the
temporary name buffer allocation isn't needed but we can directly copy
into the supplied buffer.

Tested with various getfattr -d calls on varying xattr lists.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Chris Mason <chris.mason@oracle.com>