cifs: allocate kvec array for cifs_readdata as a separate allocation

cifs: allocate kvec array for cifs_readdata as a separate allocation

Eventually, we're going to want to append a list of pages to
cifs_readdata instead of a list of kvecs. To prepare for that, turn
the kvec array allocation into a separate one and just keep a
pointer to it in the readdata.

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