LINUX: Avoid infinite d_invalidate loop

LINUX: Avoid infinite d_invalidate loop

If a reference is grabbed after we d_invalidate() a dentry, but before
we dput() that dentry, the dentry can still be on the alias list for
its inode. This will cause us to loop forever, since we'll get the
same dentry back from d_find_alias, and d_invalidate() will return
success, since it is a no-op.

Avoid this by just limiting the number of times we loop. Leaving
dentry aliases around is non-critical as long as we clear some each
time.

This change is not applicable to master, which will get a more robust
fix for this issue. This change was discussed during the 19 Dec 2012
release-team meeting.

Change-Id: Ief22ae9f8f6de62518604383c421772c25c5dc26
Reviewed-on: http://gerrit.openafs.org/8795
Reviewed-by: Paul Smeddle <paul.smeddle@gmail.com>
Reviewed-by: Stephan Wiesand <stephan.wiesand@desy.de>
Tested-by: BuildBot <buildbot@rampaginggeek.com>