[PATCH] cifs: Fix caching problem

pointed out by Dave Stahl and Vince Negri in which cifs can update the
last modify time on a server modified file without invalidating the
local cached data due to an intervening readdir. 

Signed-off-by: Steve French <(sfrench@us.ibm.com)>
Signed-off-by: Linus Torvalds <torvalds@osdl.org>