usb: gadget: f_mtp: Add support to capture time taken with vfs_write/read

This change captures time taken by each vfs_read() and vfs_write() call
made from USB MTP gadget driver where data size is equal to USB request
buffer size. It provides debugfs entry to get this information for 100
such call and resetting the same.

How-To use:
To get stats:
cat /sys/kernel/debug/usb_mtp/status
To reset stats:
echo 0 > /sys/kernel/debug/usb_mtp/status

Change-Id: I252e830568704e9e557660c1ae0f7597823e4b17
Signed-off-by: Mayank Rana <mrana@codeaurora.org>