generic_file_direct_write(): switch to iov_iter

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

diff --cc fs/btrfs/file.c
Simple merge

diff --cc fs/fuse/file.c
Simple merge

diff --cc fs/ocfs2/file.c
Simple merge

diff --cc fs/xfs/xfs_file.c
Simple merge

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

diff --cc mm/filemap.c
Simple merge