linux: defer vcache evictions when sleep would be needed

linux: defer vcache evictions when sleep would be needed

because we're only willing to loop 100 times worth of "sleeps",
on a machine with heavy vcache demands we can end up just growing
the list huge. in the first pass, just clean up as many entries which
do not require sleeping as needed. if we need more entries, make
a second pass.

Change-Id: Ie5af42e7c0287d7a093f9a5884c10813dbb8cb11
Reviewed-on: http://gerrit.openafs.org/3971
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Andrew Deason <adeason@sinenomine.net>
Reviewed-by: Derrick Brashear <shadow@dementia.org>