inode_hashtable: this is a hash table which allows lookups on sb and hash.
iget5_locked, in addition, allows an arbitrary match function (and init
function) to be passed in so that a filesystem can allow non-unique inode
numbers.

How does nfsd do lookups?

It calls sb->s_export_op->decode_fh(sb, fh, fhsize, fileid_type,
	nfsd_acceptable, exp):

where nfsd_acceptable just returns 1 in no_subtree_check case, but may return
	0 in subtree_check case if subtree check fails.  The final parameter
	of decode_fh is just a void * passed to nfsd_acceptable.

example of decode_fh:

fs/exportfs/expfs.c:export_decode_fh() is the default; it calls
	find_exported_dentry, which calls sb->s_export->get_dentry
	(get_object by default: which does export_iget, which is a fancy
	iget, doing the lookup in the inode_hastable; used by ext2/3).

iget5_locked: look up an inode by a "hash" value (usually inode number)

See __nfs_fhget for how this is used (via iget5_locked) in nfs code.