pNFS: Fix RCU lock leakage

Another brown paper bag moment. pnfs_alloc_ds_commits_list() is leaking
the RCU lock.

Fixes: a9901899

 ("pNFS: Add infrastructure for cleaning up per-layout commit structures")
Signed-off-by: Trond Myklebust <trond.myklebust@hammerspace.com>