[PATCH] lib: add idr_replace

This patch adds idr_replace() to replace an existing pointer in a single
operation.

Device-mapper will use this to update the pointer it stored against a given
id.

Signed-off-by: Jeff Mahoney <jeffm@suse.com>
Signed-off-by: Alasdair G Kergon <agk@redhat.com>
Signed-off-by: Andrew Morton <akpm@osdl.org>
Signed-off-by: Linus Torvalds <torvalds@osdl.org>