dlm: convert to idr_alloc()

Convert to the much saner new idr interface.  Error return values from
recover_idr_add() mix -1 and -errno.  The conversion doesn't change
that but it looks iffy.

Signed-off-by: Tejun Heo <tj@kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>