nfs_instantiate(): prevent multiple aliases for directory inode

Since NFS allows open-by-fhandle, we have to cope with the possibility
of mkdir vs. open-by-guessed-handle races.  A local filesystem could
decide what the inumber of the new object will be and insert a locked
inode with that inumber into icache _before_ the on-disk data structures
begin to look good and unlock it only once it has a dentry alias, so
that open-by-handle coming first would quietly fail and mkdir coming
first would have open-by-handle grab its dentry.

For NFS it's a non-starter - the icache key is server-supplied fhandle
and we do not get that until the object has been fully created on server.
We really have to deal with the possibility that open-by-handle gets
the in-core inode and attaches a dentry to it before mkdir does.

Solution: let nfs_mkdir() use d_splice_alias() to catch those.  We can
	* get an error.  Just return it to our caller.
	* get NULL - no preexisting dentry aliases, we'd just done what
d_add() would've done.  Success.
	* get a reference to preexisting alias.  In that case the alias
had been moved in place of nfs_mkdir() argument (and hashed there), while
nfs_mkdir() argument is left unhashed negative.  Which is just fine for
->mkdir() callers, all we need is to release the reference we'd got from
d_splice_alias() and report success.

Cc: Trond Myklebust <trond.myklebust@primarydata.com>
Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>