sctp: convert to idr_alloc()

sctp: convert to idr_alloc()

Convert to the much saner new idr interface.

Only compile tested.

v2: Don't preload if @gfp doesn't contain __GFP_WAIT as the function
    may be being called from non-process ocntext.  Also, add a comment
    explaining @idr_low never becomes zero.

Signed-off-by: Tejun Heo <tj@kernel.org>
Acked-by: Neil Horman <nhorman@tuxdriver.com>
Acked-by: Vlad Yasevich <vyasevich@gmail.com>
Cc: Sridhar Samudrala <sri@us.ibm.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>