xfstests: add different logging option to fsstress

xfstests: add different logging option to fsstress

Currently the only way to log fsstress's output is to redirect it's shared
stdout to pipe which is very painfull because:

1) Pipe writers are serialized via i_mutex so we waste cpu-cores power on stupid
   sinchronization for loging purpose, instead of hunting real race conditions,
   and bugs inside file system.
2) Usually output is corrupted due to luck of sychronization on shared stdout.

Since fsstress's children operate on independend paths, let's just open didicated
log file for each child and simply avoid useless sycnhronization.

Signed-off-by: Dmitry Monakhov <dmonakhov@openvz.org>
Signed-off-by: Christoph Hellwig <hch@lst.de>