With IMA-appraisal's removal of the iint mutex and taking the i_mutex
instead, allocating the iint becomes a lot simplier, as we don't need
to be concerned with two processes racing to allocate the iint. This
patch cleans up and improves performance for allocating the iint.
- removed redundant double i_mutex locking
- combined iint allocation with tree search
Changelog v2:
- removed the rwlock/read_lock changes from this patch