USB: f_mass_storage: Add support for performance recording

This patch adds support for populating the read/write performance values
for debugging the Mass storage performance numbers at VFS level.

To check the read/write values, user needs to invoke below command

cat /sys/devices/platform/usb_mass_storage/lunX/perf

To reset the read/write values to zero

echo 0 > /sys/devices/platform/usb_mass_storage/lunX/perf

Change-Id: Ied51969ce144a2335d2140259e28e8acbda44635
Signed-off-by: Chiranjeevi Velempati <cvelempa@codeaurora.org>
Signed-off-by: Rajkumar Raghupathy <raghup@codeaurora.org>