isdn/capi: check message length in capi_write()

isdn/capi: check message length in capi_write()

syzbot reported:

    BUG: KMSAN: uninit-value in capi_write+0x791/0xa90 drivers/isdn/capi/capi.c:700
    CPU: 0 PID: 10025 Comm: syz-executor379 Not tainted 4.20.0-rc7+ #2
    Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 01/01/2011
    Call Trace:
      __dump_stack lib/dump_stack.c:77 [inline]
      dump_stack+0x173/0x1d0 lib/dump_stack.c:113
      kmsan_report+0x12e/0x2a0 mm/kmsan/kmsan.c:613
      __msan_warning+0x82/0xf0 mm/kmsan/kmsan_instr.c:313
      capi_write+0x791/0xa90 drivers/isdn/capi/capi.c:700
      do_loop_readv_writev fs/read_write.c:703 [inline]
      do_iter_write+0x83e/0xd80 fs/read_write.c:961
      vfs_writev fs/read_write.c:1004 [inline]
      do_writev+0x397/0x840 fs/read_write.c:1039
      __do_sys_writev fs/read_write.c:1112 [inline]
      __se_sys_writev+0x9b/0xb0 fs/read_write.c:1109
      __x64_sys_writev+0x4a/0x70 fs/read_write.c:1109
      do_syscall_64+0xbc/0xf0 arch/x86/entry/common.c:291
      entry_SYSCALL_64_after_hwframe+0x63/0xe7
    [...]

The problem is that capi_write() is reading past the end of the message.
Fix it by checking the message's length in the needed places.

Reported-and-tested-by: syzbot+0849c524d9c634f5ae66@syzkaller.appspotmail.com
Signed-off-by: Eric Biggers <ebiggers@google.com>
Signed-off-by: David S. Miller <davem@davemloft.net>