Rename cnt to maxcookies and change its use as the condition for when to
lookup cookies to be less obscure.

No functional change.

Since r245115, cnt has not really been needed in tmpfs_dir_getdents().  Keep
it for the MPASS() for now though.

Sponsored by:	EMC / Isilon Storage Division
MFC after:	2 weeks
Add missing FALLTHROUGH comment in tmpfs_dir_getdents for looking up '.' and
'..'.

Reviewed by:	Russell Cattelan
Sponsored by:	EMC / Isilon Storage Division
MFC after:	2 weeks