new primitive: iov_iter_alignment()

returns the value aligned as badly as the worst remaining segment
in iov_iter is.  Use instead of open-coded equivalents.

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

diff --cc drivers/staging/lustre/lustre/llite/rw26.c
Simple merge

diff --cc fs/direct-io.c
Simple merge

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

diff --cc mm/iov_iter.c
Simple merge