cifs: replace kvec array in readdata with a single kvec

The array is no longer needed. We just need a single kvec to hold the
header for signature checking.

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

diff --cc fs/cifs/smb2pdu.c
Simple merge