reiserfs: Fix possible recursive lock

While allocating the bitmap using vmalloc, we hold the reiserfs lock,
which makes lockdep later reporting a possible deadlock as we may
swap out pages to allocate memory and then take the reiserfs lock
recursively:

inconsistent {RECLAIM_FS-ON-W} -> {IN-RECLAIM_FS-W} usage.
kswapd0/312 [HC0[0]:SC0[0]:HE1:SE1] takes:
 (&REISERFS_SB(s)->lock){+.+.?.}, at: [<c11108a8>] reiserfs_write_lock+0x28/0x40
{RECLAIM_FS-ON-W} state was registered at:
  [<c104e1c2>] mark_held_locks+0x62/0x90
  [<c104e28a>] lockdep_trace_alloc+0x9a/0xc0
  [<c108e396>] kmem_cache_alloc+0x26/0xf0
  [<c10850ec>] __get_vm_area_node+0x6c/0xf0
  [<c10857de>] __vmalloc_node+0x7e/0xa0
  [<c108597b>] vmalloc+0x2b/0x30
  [<c10e00b9>] reiserfs_init_bitmap_cache+0x39/0x70
  [<c10f8178>] reiserfs_fill_super+0x2e8/0xb90
  [<c1094345>] get_sb_bdev+0x145/0x180
  [<c10f5a11>] get_super_block+0x21/0x30
  [<c10931f0>] vfs_kern_mount+0x40/0xd0
  [<c10932d9>] do_kern_mount+0x39/0xd0
  [<c10a9857>] do_mount+0x2c7/0x6b0
  [<c10a9ca6>] sys_mount+0x66/0xa0
  [<c161589b>] mount_block_root+0xc4/0x245
  [<c1615a75>] mount_root+0x59/0x5f
  [<c1615b8c>] prepare_namespace+0x111/0x14b
  [<c1615269>] kernel_init+0xcf/0xdb
  [<c10031fb>] kernel_thread_helper+0x7/0x1c

This is actually fine for two reasons: we call vmalloc at mount time
then it's not in the swapping out path. Also the reiserfs lock can be
acquired recursively, but since its implementation depends on a mutex,
it's hard and not necessary worth it to teach that to lockdep.

The lock is useless at mount time anyway, at least until we replay the
journal. But let's remove it from this path later as this needs
more thinking and is a sensible change.

For now we can just relax the lock around vmalloc,

Reported-by: Alexander Beregalov <a.beregalov@gmail.com>
Signed-off-by: Frederic Weisbecker <fweisbec@gmail.com>
Cc: Chris Mason <chris.mason@oracle.com>
Cc: Ingo Molnar <mingo@elte.hu>
Cc: Thomas Gleixner <tglx@linutronix.de>