gofer: Take refs when adopting a cached endpoint-bearing inode

[copybara-service]

gofer: Take refs when adopting a cached endpoint-bearing inode

When an AF_UNIX socket is bound to a path, the resulting BoundEndpoint is
stored on the gofer inode, and other processes reach it by path: they walk
to the dentry, then pull the endpoint off its inode and connect. Crucially,
a bound socket stays connectable even when no file descriptor is holding
that path open.

The gofer refcounting invariant is: "An additional reference is held on all
synthetic dentries, and on all dentries for which endpoint is non-nil, until
they are unlinked or invalidated."

This purpose of this invariant is for a host-backed bound UDS must stay
resolvable by path even when no FD holds it. A gofer dentry at refs == 0 is
eligible for reclaim; reclaiming it tears down the inode and drops the
HostBoundEndpoint (and its host FD). So the filesystem pins every endpoint-
bearing dentry with one extra ref, held from bind until the name goes away.

This extra ref is dropped unconditionally, guarded only by endpoint != nil
in many places: unlinkAt, RenameAt, revalidation/invalidation, filesystem
teardown etc. (These drop paths assume that the invariant holds.)

To establish the invariant the extra ref is taken in the bind paths sure, but
we were missing a reference increment for a dentry that later comes to point
at the same endpoint-bearing inode. This can happen either due to a hard link
or another walk that lands on the same inode.