sunrpc: fix sleeping under rcu_read_lock in gss_stringify_acceptor

Bruce reported that he was seeing the following BUG pop:

    BUG: sleeping function called from invalid context at mm/slab.c:2846
    in_atomic(): 0, irqs_disabled(): 0, pid: 4539, name: mount.nfs
    2 locks held by mount.nfs/4539:
    #0:  (nfs_clid_init_mutex){+.+.+.}, at: [<ffffffffa01c0a9a>] nfs4_discover_server_trunking+0x4a/0x2f0 [nfsv4]
    #1:  (rcu_read_lock){......}, at: [<ffffffffa00e3185>] gss_stringify_acceptor+0x5/0xb0 [auth_rpcgss]
    Preemption disabled at:[<ffffffff81a4f082>] printk+0x4d/0x4f

    CPU: 3 PID: 4539 Comm: mount.nfs Not tainted 3.18.0-rc1-00013-g5b095e9 #3393
    Hardware name: Bochs Bochs, BIOS Bochs 01/01/2011
    ffff880021499390 ffff8800381476a8 ffffffff81a534cf 0000000000000001
    0000000000000000 ffff8800381476c8 ffffffff81097854 00000000000000d0
    0000000000000018 ffff880038147718 ffffffff8118e4f3 0000000020479f00
    Call Trace:
    [<ffffffff81a534cf>] dump_stack+0x4f/0x7c
    [<ffffffff81097854>] __might_sleep+0x114/0x180
    [<ffffffff8118e4f3>] __kmalloc+0x1a3/0x280
    [<ffffffffa00e31d8>] gss_stringify_acceptor+0x58/0xb0 [auth_rpcgss]
    [<ffffffffa00e3185>] ? gss_stringify_acceptor+0x5/0xb0 [auth_rpcgss]
    [<ffffffffa006b438>] rpcauth_stringify_acceptor+0x18/0x30 [sunrpc]
    [<ffffffffa01b0469>] nfs4_proc_setclientid+0x199/0x380 [nfsv4]
    [<ffffffffa01b04d0>] ? nfs4_proc_setclientid+0x200/0x380 [nfsv4]
    [<ffffffffa01bdf1a>] nfs40_discover_server_trunking+0xda/0x150 [nfsv4]
    [<ffffffffa01bde45>] ? nfs40_discover_server_trunking+0x5/0x150 [nfsv4]
    [<ffffffffa01c0acf>] nfs4_discover_server_trunking+0x7f/0x2f0 [nfsv4]
    [<ffffffffa01c8e24>] nfs4_init_client+0x104/0x2f0 [nfsv4]
    [<ffffffffa01539b4>] nfs_get_client+0x314/0x3f0 [nfs]
    [<ffffffffa0153780>] ? nfs_get_client+0xe0/0x3f0 [nfs]
    [<ffffffffa01c83aa>] nfs4_set_client+0x8a/0x110 [nfsv4]
    [<ffffffffa0069708>] ? __rpc_init_priority_wait_queue+0xa8/0xf0 [sunrpc]
    [<ffffffffa01c9b2f>] nfs4_create_server+0x12f/0x390 [nfsv4]
    [<ffffffffa01c1472>] nfs4_remote_mount+0x32/0x60 [nfsv4]
    [<ffffffff81196489>] mount_fs+0x39/0x1b0
    [<ffffffff81166145>] ? __alloc_percpu+0x15/0x20
    [<ffffffff811b276b>] vfs_kern_mount+0x6b/0x150
    [<ffffffffa01c1396>] nfs_do_root_mount+0x86/0xc0 [nfsv4]
    [<ffffffffa01c1784>] nfs4_try_mount+0x44/0xc0 [nfsv4]
    [<ffffffffa01549b7>] ? get_nfs_version+0x27/0x90 [nfs]
    [<ffffffffa0161a2d>] nfs_fs_mount+0x47d/0xd60 [nfs]
    [<ffffffff81a59c5e>] ? mutex_unlock+0xe/0x10
    [<ffffffffa01606a0>] ? nfs_remount+0x430/0x430 [nfs]
    [<ffffffffa01609c0>] ? nfs_clone_super+0x140/0x140 [nfs]
    [<ffffffff81196489>] mount_fs+0x39/0x1b0
    [<ffffffff81166145>] ? __alloc_percpu+0x15/0x20
    [<ffffffff811b276b>] vfs_kern_mount+0x6b/0x150
    [<ffffffff811b5830>] do_mount+0x210/0xbe0
    [<ffffffff811b54ca>] ? copy_mount_options+0x3a/0x160
    [<ffffffff811b651f>] SyS_mount+0x6f/0xb0
    [<ffffffff81a5c852>] system_call_fastpath+0x12/0x17

Sleeping under the rcu_read_lock is bad. This patch fixes it by dropping
the rcu_read_lock before doing the allocation and then reacquiring it
and redoing the dereference before doing the copy. If we find that the
string has somehow grown in the meantime, we'll reallocate and try again.

Cc: <stable@vger.kernel.org> # v3.17+
Reported-by: "J. Bruce Fields" <bfields@fieldses.org>
Signed-off-by: Jeff Layton <jlayton@primarydata.com>
Signed-off-by: Trond Myklebust <trond.myklebust@primarydata.com>