cifs: get rid of ->f_path.dentry->d_sb uses, add a new helper

Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>

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

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

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

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