inode: prevent plgfs_alloc_ii() sleeping in atomic context

plgfs_alloc_ii() may be called from atomic context. iget5_locked() performs
plgfs_inode_set() while holding inode_hash_lock spin lock, thus kmem_cache_alloc()
should not see __GFP_WAIT bit:

 BUG: sleeping function called from invalid context at mm/slub.c:950
 in_atomic(): 1, irqs_disabled(): 0, pid: 2061, name: iozone
 CPU: 3 PID: 2061 Comm: iozone Not tainted 3.13.0-rc7-next-20140107-dbg-dirty #94
 Call Trace:
  [<ffffffff8148e79b>] dump_stack+0x4e/0x7a
  [<ffffffff8106fd4b>] __might_sleep+0x1a7/0x1ae
  [<ffffffff81131fef>] kmem_cache_alloc+0x48/0x1cf
  [<ffffffffa02ffe0d>] ? plgfs_alloc_ii+0x1b/0x2e [pluginfs]
  [<ffffffffa02fe569>] ? plgfs_alloc_di+0x45/0x45 [pluginfs]
  [<ffffffffa02ffe0d>] plgfs_alloc_ii+0x1b/0x2e [pluginfs]
  [<ffffffffa02ffe42>] plgfs_inode_set+0x22/0xd8 [pluginfs]
  [<ffffffff8115696a>] iget5_locked+0xc6/0x1a5
  [<ffffffffa02ffe20>] ? plgfs_alloc_ii+0x2e/0x2e [pluginfs]
  [<ffffffffa02ff36b>] plgfs_iget+0x44/0x98 [pluginfs]
  [<ffffffffa02ffbdf>] plgfs_dir_iop_create+0xf8/0x1ce [pluginfs]
  [<ffffffff81148bb5>] vfs_create+0x8b/0xc4
  [<ffffffff8114ac0c>] do_last.isra.43+0x4fd/0xa13
  [<ffffffff8114b335>] path_openat+0x213/0x5ca
  [<ffffffff8114c41b>] do_filp_open+0x3a/0x7f
  [<ffffffff81495081>] ? _raw_spin_unlock+0x31/0x45
  [<ffffffff811597c4>] ? __alloc_fd+0x1b1/0x1c3
  [<ffffffff8113d91b>] do_sys_open+0x14d/0x1dc
  [<ffffffff8100f249>] ? syscall_trace_enter+0x178/0x229
  [<ffffffff8113d9c8>] SyS_open+0x1e/0x20
  [<ffffffff81496996>] tracesys+0xd4/0xd9

Signed-off-by: Sergey Senozhatsky <sergey.senozhatsky@gmail.com>