reiserfs: Safely acquire i_mutex from xattr_rmdir

Relax the reiserfs lock before taking the inode mutex from
xattr_rmdir() to avoid the usual reiserfs lock <-> inode mutex
bad dependency.

Signed-off-by: Frederic Weisbecker <fweisbec@gmail.com>
Tested-by: Christian Kujau <lists@nerdbynature.de>
Cc: Alexander Beregalov <a.beregalov@gmail.com>
Cc: Chris Mason <chris.mason@oracle.com>
Cc: Ingo Molnar <mingo@elte.hu>