NFS: Fix a request reference leak in nfs_direct_write_clear_reqs()

nfs_direct_write_scan_commit_list() will lock the request and bump
the reference count, but we also need to account for the reference
that was taken when we initially added the request to the commit list.

Fixes: fb5f7f20 ("NFS: commit errors should be fatal")
Signed-off-by: Trond Myklebust <trond.myklebust@hammerspace.com>