nfs: Annotate struct pnfs_commit_array with __counted_by()

Add the __counted_by compiler attribute to the flexible array member
buckets to improve access bounds-checking via CONFIG_UBSAN_BOUNDS and
CONFIG_FORTIFY_SOURCE.

Compile-tested only.

Signed-off-by: Thorsten Blum <thorsten.blum@linux.dev>
Signed-off-by: Trond Myklebust <trond.myklebust@hammerspace.com>