Linux: mmap: Apply recursion check only to recursion cases

Linux: mmap: Apply recursion check only to recursion cases

The CPageWrite flag was originally added to prevent a scenario
where a thread doing "writepage" would realize that the cache
was too full and that some of its contents need to be written
back to the server.  Before writing back it would ask the OS to
flush any dirty VM associated with the vcache entries that are
to be written, to make sure the data is not stale.  This flush
could itself trigger writeback, leading to deadly recursion.
One such scenario is a process doing mmap writes to a file larger
than the cache.

With some kernel versions and some callers of writepage, this
can cause the mapping to be marked as being in an error state,
leading to EIO errors passed back to user space.

Make the recursion check more specific to only bail when the
calling thread is one that was originally seen writing.  A list
of current writers is maintained instead of a single state flag.

This lets other threads (like the flusher thread) go on with
writeback to the same file, and limits the WRITEPAGE_ACTIVATE
return case to call sites that can deal with it.

In testing this helps avoid EIO errors when writing large
chunks of data through mmap.

Thanks to Yadav Yadavendra for extensive analysis and testing.

Change-Id: Ic3136d7050c62e3ffac5e52441171f322b60fe86
Reviewed-on: http://gerrit.openafs.org/11124
Reviewed-by: Daria Brashear <shadow@your-file-system.com>
Tested-by: BuildBot <buildbot@rampaginggeek.com>