misc: fastrpc: fix list iterator in fastrpc_req_mem_unmap_impl

In the Linux kernel, the following vulnerability has been resolved:

misc: fastrpc: fix list iterator in fastrpc_req_mem_unmap_impl

This is another instance of incorrect use of list iterator and
checking it for NULL.

The list iterator value 'map' will *always* be set and non-NULL
by list_for_each_entry(), so it is incorrect to assume that the
iterator value will be NULL if the list is empty (in this case, the
check 'if (!map) {' will always be false and never exit as expected).

To fix the bug, use a new variable 'iter' as the list iterator,
while use the original variable 'map' as a dedicated pointer to
point to the found element.

Without this patch, Kernel crashes with below trace:

Unable to handle kernel access to user memory outside uaccess routines
at virtual address 0000ffff7fb03750
...
Call trace:
fastrpc_map_create+0x70/0x290 [fastrpc]
fastrpc_req_mem_map+0xf0/0x2dc [fastrpc]
fastrpc_device_ioctl+0x138/0xc60 [fastrpc]
__arm64_sys_ioctl+0xa8/0xec
invoke_syscall+0x48/0x114
el0_svc_common.constprop.0+0xd4/0xfc
do_el0_svc+0x28/0x90
el0_svc+0x3c/0x130
el0t_64_sync_handler+0xa4/0x130
el0t_64_sync+0x18c/0x190
Code: 14000016 f94000a5 eb05029f 54000260 (b94018a6)
---[ end trace 0000000000000000 ]---