Partial fix for tn_dirent_hint issue.

tmpfs_dir_detach always disassociated the dirent from its node. This is
problematic in that a file can have more than one directory entries
(hardlinks) and tn_dirent_hint could be set to another dirent. Only set
to NULL if tn_dirent_hint equals the dirent to be detached.

XXX: tn_dirent_hint should be set to NULL if and only if ther is no
directory entry left. This still has to be fixed!