switch vfs_mkdir() and ->mkdir() to umode_t

vfs_mkdir() gets int, but immediately drops everything that might not
fit into umode_t and that's the only caller of ->mkdir()...

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

Conflicts:
	fs/affs/affs.h
	fs/cifs/cifsfs.h
	fs/nfs/dir.c
	fs/xfs/xfs_iops.c
	include/linux/fs.h