misc: fastrpc: fix use-after-free race in fastrpc_map_create

fastrpc_map_lookup returns a raw pointer after releasing fl->lock. The
caller fastrpc_map_create then calls fastrpc_map_get (kref_get_unless_zero)
on this unprotected pointer. A concurrent MEM_UNMAP can free the map
between the lock release and the kref operation, resulting in a
use-after-free on the freed slab object.

Restore the take_ref parameter to fastrpc_map_lookup so the reference
is acquired atomically under fl->lock before the pointer is exposed to
the caller.

Fixes: 10df039834f8 ("misc: fastrpc: Skip reference for DMA handles")
Cc: stable@vger.kernel.org
Signed-off-by: Zhenghang Xiao <kipreyyy@gmail.com>
Signed-off-by: Srinivas Kandagatla <srini@kernel.org>
Link: https://patch.msgid.link/20260530204528.116920-5-srini@kernel.org
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>