vfs: introduce d_instantiate_no_diralias()

...which just returns -EBUSY if a directory alias would be created.

This is to be used by fuse mkdir to make sure that a buggy or malicious
userspace filesystem doesn't do anything nasty.  Previously fuse used a
private mutex for this purpose, which can now go away.

Signed-off-by: Miklos Szeredi <mszeredi@suse.cz>

diff --cc fs/dcache.c
Simple merge

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

diff --cc fs/fuse/fuse_i.h
Simple merge

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

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