usb: gadget: f_mtp: Securing vfs_write and unbind with less mutex

While doing mtp file transfers and performing usb disconnect,
due to usb request being shared between both vfs_write and
unbind, creates a race condition, result in crashing the kernel.

Fix this by introducing mutex in such a ways that it avoids buffer
being freed and also do not degrade the throughput while copy
operation is performed.

Change-Id: Ic235fb92997993bd75ed5e519a5d73e787522810
Signed-off-by: Udipto Goswami <ugoswami@codeaurora.org>
Signed-off-by: Manu Gautam <mgautam@codeaurora.org>