ramfs support.

I've seen this work *once*. Then it stopped working. Now it gets panics.

To test:
ramfs -s &
mount -c '#s/ramfs' /mnt
>/9/mnt/x

And see this:

0x0000000000000003: sysopen /mnt/x mode 1101
0x0000000000000002: sysread 3 0x0000000000611104 16 -1
ramfs 2:<-Twalk tag 1d fid 24 newfid 28 nwname 1 0:x
ramfs 2:->Rerror tag 1d ename file does not exist
0x0000000000000002: syswrite 3 0x0000000000611100 28 -1
0x0000000000000002: sysread 3 0x0000000000611100 4 -1
0x0000000000000002: sysread 3 0x0000000000611104 13 -1
ramfs 2:<-Twalk tag 1d fid 24 newfid 30 nwname 0
ramfs 2:->Rwalk tag 1d nwqid 0d
0x0000000000000002: syswrite 3 0x0000000000611100 9 -1
0x0000000000000002: sysread 3 0x0000000000611100 4 -1
0x0000000000000002: sysread 3 0x0000000000611104 15 -1
[00000002] user READ fault va 0x000000000000001c ip 0x000000000040b147 on core 0 with err -14
HW TRAP frame at 0xffff8000bea6bf40 on core 0
  rax  0x00007f7fffbf9810
  rbx  0x00007f7fffbfd970
  rcx  0x0000000000688810
  rdx  0x00007f7fffbf9790
  rbp  0x00007f7fffbfa7a0
  rsi  0x0000000000000000
  rdi  0x00007f7fffbfd970
  r8   0x0000000000000001
  r9   0x0000000000000003
  r10  0x000000000000004d
  r11  0x00007f7fffbf97e0
  r12  0x00007f7fffbfa7b8
  r13  0x000000000040d258
  r14  0x0000000000000023
  r15  0x0000000000689320
  trap 0x0000000e Page Fault
  gsbs 0x0000000000000000
  fsbs 0x00004000005cb060
  err  0x--------00000004
  rip  0x000000000040b147
  cs   0x------------0023
  flag 0x0000000000000206
  rsp  0x00007f7fffbf96f0
  ss   0x------------001b
HW TRAP frame at 0xffff8000befdd7f0 on core 0
  rax  0x0000000000000000
  rbx  0x0000000000000000
  rcx  0xffff8000befdd8f8
  rdx  0x0000000000000000
  rbp  0xffff8000befdd8c8
  rsi  0xffff8000befdd8e7
  rdi  0x0000000000000000
  r8   0x0000000000000000
  r9   0x0000000000000002
  r10  0xffff8000000b8f00
  r11  0xffff8000000b8ec0
  r12  0xffff8000be635130
  r13  0xffff8000bea94560
  r14  0x0000000000000000
  r15  0x00000000ffffffff
  trap 0x0000000e Page Fault
  gsbs 0xffffffffc2962c40
  fsbs 0x00000000deadbeef
  err  0x--------00000000
  rip  0xffffffffc030a510
  cs   0x------------0008
  flag 0x0000000000000046
  rsp  0xffff8000befdd8b8
  ss   0x------------0000

Backtrace of faulting kernel context on Core 0:
kernel panic at kern/arch/x86/trap64.c:128, from core 0: Page Fault in the Kernel at 0x00000014!
Entering Nanwan's Dungeon on Core 0 (Ints off):

I added the spinlock_init in dev/mnt.c but that Helpeth Not.

Signed-off-by: Ronald G. Minnich <rminnich@google.com>