tools :acrntrace: Reserved configurable space on the disk

acrntrace writes trace data to /tmp with unlimited size and will cause tmpfs
100% occupied. Consequently, some problem will raise up on SOS, like failing to
exec cmd.

This patch introduce an option -r to let user configre minimal space left on the
disk which ensures that acrntrace will exit when free storage space is less than
the reserved space. By default, we reserve 512M on the disk. Users can configure
reserved space through '-r'.

Signed-off-by: Kaige Fu <kaige.fu@intel.com>
Reviewed-by: Eddie Dong <eddie.dong@intel.com>
Reviewed-by: Geoffroy Van Cutsem <geoffroy.vancutsem@intel.com>

tools/acrntrace/README.rst

@@ -20,6 +20,8 @@ Options:
 -h			print this message
 -t period		specify polling interval in milliseconds [1-999]
 -c			clear the buffered old data
+-r			minimal amount (in MB) of free space kept on the disk
+			before acrntrace stops
 
 The ``acrntrace_format.py`` is a offline tool for parsing trace data (as output
 by acrntrace) to human-readable formats based on given format.
@@ -89,6 +91,10 @@ data to your linux system, and running the analysis tool.
 
       # acrntrace
 
+   512MB storage space will be reserved by default. This ensures that acrntrace
+   will exit automatically when the free storage space on the disk is less than
+   reserved space. Reserved space on the disk is configurable through '-r'.
+
    Trace files are created under ``/tmp/acrntrace/``, with a
    date-time-based directory name such as ``20171115-101605``