[RFC][PATCHSET] reduce messing with iovecs in cifs

[Al Viro]

	Now that sendmsg/recvmsg do not mangle iovecs and are capable of
handling bvec-based ->msg_iter, we can seriously reduce the amount of PITA
in cifs.  The series below is completely untested, and I would appreciate
comments/review/testing/etc.

I'll post the individual patches in followups; for those who prefer to use
git it can be found in
git://git.kernel.org/pub/scm/linux/kernel/git/viro/vfs.git sendmsg.cifs

1/6: [net] drop 'size' argument of sock_recvmsg()
	should go via net-next; does what it says.

2/6: cifs: merge the hash calculation helpers
	takes the common parts of {cifs,smb2,smb3}_calc_signature() into a
common helper.

3/6: cifs: quit playing games with draining iovecs
	Switch smb_send_kvec() to passing msghdr (and thus iov_iter) and
make it use sock_sendmsg() - that allows to avoid draining iovecs, since
->msg_iter will be advanced properly and all we need is to keep it around
between the calls of sock_sendmsg(), rather than reinitializing it on each
loop iteration.  The same thing allows to get rid of messing with kmap()
when sending the stuff in ->rq_pages[] - ITER_BVEC will do the right thing.

4/6: cifs: no need to wank with copying and advancing iovec on recvmsg side either
	Similar to the previous - use sock_recvmsg() in cifs_readv_from_socket()
and there's no need to modify iovecs, or allocate a copy especially for
such modifications, etc.

5/6: cifs_readv_receive: use cifs_read_from_socket()
	building a 1-element iovec array for cifs_readv_from_socket() is
an overkill - simple cifs_read_from_socket() will do just fine.

6/6: cifs: don't bother with kmap on read_pages side
	Similar to the other half of 3/6: we can use ITER_BVEC for
read-into-page case.  Just make cifs_readv_from_socket() take msghdr from
caller and use a helper that would feed it a bvec-backed ->msg_iter.