Revert "um: ubd: Set device serial attribute from cmdline"

This reverts commit ef3ba87c.

kernel_test build is broken by this change:

<5>Kernel command line: mem=512M ubdar=/home/paullawrence/code/kernels/common/kernel/tests/net/test/net_test.rootfs.20150203 con=null,fd:1 ro panic=1 init=/sbin/net_test.sh net_test_args="" net_test_mode=builder random.trust_cpu=on entropy=sPiFf7S2QnyBKuFrKmhuG9T441Gxu0E6taYRZ6cz0dNVup/2u7RHlr0BhlH+zn60 net_test=/host/home/paullawrence/code/kernels/common/kernel/tests/net/test/all_tests.sh net_test_exitcode=/proc/exitcode root=98:0
<4>
<4>Modules linked in:
<6>Pid: 0, comm: swapper Not tainted 5.11.0-rc4-02525-g902943e0329c
<6>RIP: 0033:[<0000000060032853>]
<6>RSP: 00000000605f3db0  EFLAGS: 00010246
<6>RAX: 0000000000000000 RBX: 0000000000000000 RCX: 000000006054efd3
<6>RDX: 0000000000000000 RSI: 000000006054efd0 RDI: 00000000605f3dd8
<6>RBP: 00000000605f3e10 R08: 0000000000000000 R09: 000000000000002c
<6>R10: 0000000081756c64 R11: 0000000000000000 R12: 0000000000000000
<6>R13: 00000000605e0981 R14: 00000000605f3e20 R15: 00000000605fbab8
<0>Kernel panic - not syncing: Segfault with no mm
<4>CPU: 0 PID: 0 Comm: swapper Not tainted 5.11.0-rc4-02525-g902943e0329c #1
<4>Stack:
<4> 00000000 00000000 00000000 605fba90
<4> 81756c0f 00000000 60618d78 81756c0c
<4> 81756c0c 605e09d8 600601e4 81756c09
<4>Call Trace:
<4> [<600601e4>] ? parameqn+0x0/0x41
<4> [<60031e98>] ubd_setup+0x1b/0x4c
<4> [<60031e7d>] ? ubd_setup+0x0/0x4c
<4> [<60001dda>] obsolete_checksetup+0x82/0xc2
<4> [<60001d58>] ? obsolete_checksetup+0x0/0xc2
<4> [<60001888>] ? unknown_bootoption+0x0/0x168
<4> [<600018cc>] unknown_bootoption+0x44/0x168
<4> [<60001888>] ? unknown_bootoption+0x0/0x168
<4> [<60060225>] ? parameq+0x0/0x66
<4> [<60060382>] parse_args+0xf7/0x2af
<4> [<6006028b>] ? parse_args+0x0/0x2af
<4> [<6007d60e>] ? printk+0x0/0x7f
<4> [<60039e20>] ? block_signals+0x0/0x13
<4> [<60001459>] start_kernel+0x182/0x4de
<4> [<60001888>] ? unknown_bootoption+0x0/0x168
<4> [<60003aae>] start_kernel_proc+0x49/0x51
<4> [<60025a3d>] new_thread_handler+0x81/0xae
<4> [<60028981>] uml_finishsetup+0x54/0x59
/home/paullawrence/code/kernels/common/build/../kernel/tests/net/test/run_net_test.sh: line 431: 1137943 Aborted                 $KERNEL_BINARY umid=net_test mem=512M $blockdevice=$ROOTFS $netconfig $consolemode $cmdline 1>&2
Returning exit code 134.

Bug: 177651696
Test: Local build of kernel_test fails with the original change,
      succeeds when the change is reverted
Signed-off-by: Paul Lawrence <paullawrence@google.com>
Change-Id: I15b274ee83b3bf8c53a3ff79e73f0864014e8034