cifs: ensure that we always do cifsFileInfo_get under the spinlock

The readpages bug is a regression that was introduced in 6993f74a.
This also fixes a couple of similar bugs in the uncached read and write
codepaths.

Also, prevent this sort of thing in the future by having cifsFileInfo_get
take the spinlock itself, and adding a _locked variant for use in places
that are already holding the lock. The _put code has always done that
so this makes for a less confusing interface.

Cc: <stable@vger.kernel.org> # 3.5.x
Reviewed-by: Pavel Shilovsky <pshilovsky@samba.org>
Signed-off-by: Jeff Layton <jlayton@redhat.com>
Signed-off-by: Steve French <smfrench@gmail.com>