[CIFS] Fix spurious reconnect on 2nd peek from read of SMB length

When retrying kernel_recvmsg() because of a short read, check returned
length against the remaining length, not against total length. This
avoids unneeded session reconnects which would otherwise occur when
kernel_recvmsg() finally returns zero when asked to read zero bytes.

Signed-off-by: Petr Tesarik <ptesarik@suse.cz>
Signed-off-by: Jeff Layton <jlayton@redhat.com>
Signed-off-by: Steve French <sfrench@us.ibm.com>