new helper: iov_iter_bvec()

similar to iov_iter_kvec(), for ITER_BVEC ones

Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>

diff --cc fs/splice.c
Simple merge

diff --cc include/linux/uio.h
Simple merge

diff --cc mm/iov_iter.c
Simple merge

diff --cc mm/page_io.c
Simple merge