[PATCH] VFS: split dentry locking documentation

This patch splits dentry locking documentation from
Documentation/filesystems/vfs.txt to a separate file.  The dentry locking
bits are useful but do not fit into the VFS overview document as is.

Signed-off-by: Pekka Enberg <penberg@cs.helsinki.fi>
Signed-off-by: Andrew Morton <akpm@osdl.org>
Signed-off-by: Linus Torvalds <torvalds@osdl.org>