kill configfs_drop_dentry()

Fold into the only remaining user, don't bother with the timestamps
of parent - we are going to rmdir it shortly anyway, which will
override those.

Fix the locking of inode, while we are at it - updating the link
count and timestamps ought to be done with the inode locked.

Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>