Update tmpfs link count handling for ino64.

Add a new TMPFS_LINK_MAX to use in place of LINK_MAX for link overflow
checks and pathconf() reporting. Rather than storing a full 64-bit
link count, just use a plain int and use INT_MAX as TMPFS_LINK_MAX.

Discussed with: bde
Reviewed by: kib (part of a larger patch)
Sponsored by: Chelsio Communications