NFSv4.1: Hold reference to layout hdr in layoutget

This fixes an oops where a LAYOUTGET is in still in the rpciod queue,
but the requesting processes has been killed.  Without this, killing
the process does the final pnfs_put_layout_hdr() and sets NFS_I(inode)->layout
to NULL while the LAYOUTGET rpc task still references it.

Example oops:

BUG: unable to handle kernel NULL pointer dereference at 0000000000000080
IP: [<ffffffffa01bd586>] pnfs_choose_layoutget_stateid+0x37/0xef [nfsv4]
PGD 7365b067 PUD 7365d067 PMD 0
Oops: 0000 [#1] SMP DEBUG_PAGEALLOC
Modules linked in: nfs_layout_nfsv41_files nfsv4 auth_rpcgss nfs lockd sunrpc ipt_MASQUERADE ip6table_mangle ip6t_REJECT nf_conntrack_ipv6 nf_defrag_ipv6 iptable_nat nf_nat_ipv4 nf_nat iptable_mangle ip6table_filter ip6_tables ppdev e1000 i2c_piix4 i2c_core shpchp parport_pc parport crc32c_intel aesni_intel xts aes_x86_64 lrw gf128mul ablk_helper cryptd mptspi scsi_transport_spi mptscsih mptbase floppy autofs4
CPU 0
Pid: 27, comm: kworker/0:1 Not tainted 3.8.0-dros_cthon2013+ #4 VMware, Inc. VMware Virtual Platform/440BX Desktop Reference Platform
RIP: 0010:[<ffffffffa01bd586>]  [<ffffffffa01bd586>] pnfs_choose_layoutget_stateid+0x37/0xef [nfsv4]
RSP: 0018:ffff88007b0c1c88  EFLAGS: 00010246
RAX: ffff88006ed36678 RBX: 0000000000000000 RCX: 0000000ea877e3bc
RDX: ffff88007a729da8 RSI: 0000000000000000 RDI: ffff88007a72b958
RBP: ffff88007b0c1ca8 R08: 0000000000000002 R09: 0000000000000000
R10: 0000000000000000 R11: 0000000000000000 R12: ffff88007a72b958
R13: ffff88007a729da8 R14: 0000000000000000 R15: ffffffffa011077e
FS:  0000000000000000(0000) GS:ffff88007f600000(0000) knlGS:0000000000000000
CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
CR2: 0000000000000080 CR3: 00000000735f8000 CR4: 00000000001407f0
DR0: 0000000000000000 DR1: 0000000000000000 DR2: 0000000000000000
DR3: 0000000000000000 DR6: 00000000ffff0ff0 DR7: 0000000000000400
Process kworker/0:1 (pid: 27, threadinfo ffff88007b0c0000, task ffff88007c2fa0c0)
Stack:
 ffff88006fc05388 ffff88007a72b908 ffff88007b240900 ffff88006fc05388
 ffff88007b0c1cd8 ffffffffa01a2170 ffff88007b240900 ffff88007b240900
 ffff88007b240970 ffffffffa011077e ffff88007b0c1ce8 ffffffffa0110791
Call Trace:
 [<ffffffffa01a2170>] nfs4_layoutget_prepare+0x7b/0x92 [nfsv4]
 [<ffffffffa011077e>] ? __rpc_atrun+0x15/0x15 [sunrpc]
 [<ffffffffa0110791>] rpc_prepare_task+0x13/0x15 [sunrpc]

Reported-by: Tigran Mkrtchyan <tigran.mkrtchyan@desy.de>
Signed-off-by: Weston Andros Adamson <dros@netapp.com>
Cc: stable@kernel.org
Signed-off-by: Trond Myklebust <Trond.Myklebust@netapp.com>