misc: fastrpc: Add missing lock before accessing find_vma()

fastrpc driver is using find_vma() without any protection, as a
result we see below warning due to recent patch 5b78ed24e8
("mm/pagemap: add mmap_assert_locked() annotations to find_vma*()")
which added mmap_assert_locked() in find_vma() function.

This bug went un-noticed in previous versions. Fix this issue by adding
required protection while calling find_vma().

CPU: 0 PID: 209746 Comm: benchmark_model Not tainted 5.15.0-rc2-00445-ge14fe2bf817a-dirty #969
Hardware name: Qualcomm Technologies, Inc. Robotics RB5 (DT)
pstate: 60400005 (nZCv daif +PAN -UAO -TCO -DIT -SSBS BTYPE=--)
pc : find_vma+0x64/0xd0
lr : find_vma+0x60/0xd0
sp : ffff8000158ebc40
...

Call trace:
 find_vma+0x64/0xd0
 fastrpc_internal_invoke+0x570/0xda8
 fastrpc_device_ioctl+0x3e0/0x928
 __arm64_sys_ioctl+0xac/0xf0
 invoke_syscall+0x44/0x100
 el0_svc_common.constprop.3+0x70/0xf8
 do_el0_svc+0x24/0x88
 el0_svc+0x3c/0x138
 el0t_64_sync_handler+0x90/0xb8
 el0t_64_sync+0x180/0x184

Fixes: 80f3afd72b ("misc: fastrpc: consider address offset before sending to DSP")
Cc: stable@vger.kernel.org
Signed-off-by: Srinivas Kandagatla <srinivas.kandagatla@linaro.org>
Link: https://lore.kernel.org/r/20210922154326.8927-1-srinivas.kandagatla@linaro.org
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

drivers/misc/fastrpc.c

@@ -814,10 +814,12 @@ static int fastrpc_get_args(u32 kernel, struct fastrpc_invoke_ctx *ctx)
 			rpra[i].pv = (u64) ctx->args[i].ptr;
 			pages[i].addr = ctx->maps[i]->phys;
 
+			mmap_read_lock(current->mm);
 			vma = find_vma(current->mm, ctx->args[i].ptr);
 			if (vma)
 				pages[i].addr += ctx->args[i].ptr -
 						 vma->vm_start;
+			mmap_read_unlock(current->mm);
 
 			pg_start = (ctx->args[i].ptr & PAGE_MASK) >> PAGE_SHIFT;
 			pg_end = ((ctx->args[i].ptr + len - 1) & PAGE_MASK) >>