Fix "fuse internal error: node NNN not found"

Commit 4dc7e675bb (Don't unhash name in FORGET) broke the forget logic in a
subtle way, resulting in "fuse internal error: node NNN not found" and causing
the filesystem daemon to abort.

Fix by incrementing the node refcount if nlookup goes from zero to one.

Reported by Kyle Lippincott
2 files changed