omfs: stop playing silly buggers with omfs_unlink() in ->rename()

Since omfs directories are hashes of inodes and name is part of
inode, we have to remove inode from old directory before we can
put it into new one / under new name.  So instead of
	bump i_nlink
	call omfs_unlink, which does
		omfs_delete_entry()
		decrement i_nlink and mark parent dirty in case of success
	decrement i_nlink if omfs_unlink failed and hadn't done it itself
let's just call omfs_delete_entry() and dirty the parent ourselves...

Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>
Signed-off-by: Bob Copeland <me@bobcopeland.com>