Fuse considered it an error (EIO) if lookup returned a directory inode, to
which a dentry already refered.  This is because directory aliases are not
allowed.

But in a network filesystem this could happen legitimately, if a directory is
moved on a remote client.  This patch attempts to relax the restriction by
trying to first evict the offending alias from the cache.  If this fails, it
still returns an error (EBUSY).

A rarer situation is if an mkdir races with an indenpendent lookup, which
finds the newly created directory already moved.  In this situation the mkdir
should return success, but that would be incorrect, since the dentry cannot be
instantiated, so return EBUSY.

Previously checking for a directory alias and instantiation of the dentry
weren't done atomically in lookup/mkdir, hence two such calls racing with each
other could create aliased directories.  To prevent this introduce a new
per-connection mutex: fuse_conn->inst_mutex, which is taken for instantiations
with a directory inode.
Signed-off-by: Miklos Szeredi <miklos@szeredi.hu>
Signed-off-by: Andrew Morton <akpm@osdl.org>
Signed-off-by: Linus Torvalds <torvalds@osdl.org>