LU-2304 tests: fsx log print file descriptor number

LU-2304 tests: fsx log print file descriptor number

Print the file descriptor number when dumping logs after an fsx
failure during multi-mount testing.  This allows easier tracking
of which mountpoint data was written to and read from.

Signed-off-by: Andreas Dilger <adilger@whamcloud.com>
Change-Id: I5800bb982e1874d2394a554c95289342ab3444a2
Reviewed-on: http://review.whamcloud.com/4498
Tested-by: Hudson
Tested-by: Maloo <whamcloud.maloo@gmail.com>
Reviewed-by: Jinshan Xiong <jinshan.xiong@whamcloud.com>
Reviewed-by: Yu Jian <yujian@whamcloud.com>
Reviewed-by: Oleg Drokin <green@whamcloud.com>