hfsplus: switch to ->iterate_shared()

We need to protect the list of hfsplus_readdir_data against parallel
insertions (in readdir) and removals (in release).  Add a spinlock
for that.  Note that it has nothing to do with protection of
hfsplus_readdir_data->key - we have an exclusion between hfsplus_readdir()
and hfsplus_delete_cat() on directory lock and between several
hfsplus_readdir() for the same struct file on ->f_pos_lock.  The spinlock
is strictly for list changes.

Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>