cifs: call cifs_update_eof with i_lock held

cifs_update_eof has the potential to be racy if multiple threads are
trying to modify it at the same time. Protect modifications of the
server_eof value with the inode->i_lock.

Signed-off-by: Jeff Layton <jlayton@redhat.com>

diff --cc fs/cifs/cifsglob.h
Simple merge

diff --cc fs/cifs/cifssmb.c
Simple merge

diff --cc fs/cifs/file.c
Simple merge