minix: move releasing pages into unlink and rename

Instead of consuming the page reference and kmap in the low-level
minix_delete_entry and minix_set_link helpers, do it in the callers
where that code can be shared with the error cleanup path.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>