Prevent potential double frees in sg driver

sg_ioctl could be spammed by requests, leading to a double free in
__free_pages. This protects the entry points of sg_ioctl where the
memory could be corrupted by a double call to __free_pages if multiple
requests are happening concurrently.

Bug:35644812

Change-Id: Ie13f65beb6974430f90292e2742841b26aecb8b1
Signed-off-by: Robb Glasser <rglasser@google.com>
Git-commit: 22d8e80738b5ce8784d59b48b0b051a520da4bec
Git-repo: https://android.googlesource.com/kernel/msm


[srkupp@codeaurora.org: Resolved trivial conflicts]
Signed-off-by: Srinivasa Rao Kuppala <srkupp@codeaurora.org>
Signed-off-by: Sayali Lokhande <sayalil@codeaurora.org>