mremap: remove LATENCY_LIMIT from mremap to reduce the number of TLB shootdowns

Commit 5d190420 ("mremap: fix race between mremap() and page
cleanning") fixed races between mremap and other operations for both
file-backed and anonymous mappings.  The file-backed was the most
critical as it allowed the possibility that data could be changed on a
physical page after page_mkclean returned which could trigger data loss
or data integrity issues.

A customer reported that the cost of the TLBs for anonymous regressions
was excessive and resulting in a 30-50% drop in performance overall
since this commit on a microbenchmark.  Unfortunately I neither have
access to the test-case nor can I describe what it does other than
saying that mremap operations dominate heavily.

This patch removes the LATENCY_LIMIT to handle TLB flushes on a PMD
boundary instead of every 64 pages to reduce the number of TLB
shootdowns by a factor of 8 in the ideal case.  LATENCY_LIMIT was almost
certainly used originally to limit the PT...