selinux: fix sleeping function called from invalid context

selinux_sb_mnt_opts_compat() is called via sget_fc() under the sb_lock
spinlock, so it can't use GFP_KERNEL allocations:

[  868.565200] BUG: sleeping function called from invalid context at
               include/linux/sched/mm.h:230
[  868.568246] in_atomic(): 1, irqs_disabled(): 0,
               non_block: 0, pid: 4914, name: mount.nfs
[  868.569626] preempt_count: 1, expected: 0
[  868.570215] RCU nest depth: 0, expected: 0
[  868.570809] Preemption disabled at:
[  868.570810] [<0000000000000000>] 0x0
[  868.571848] CPU: 1 PID: 4914 Comm: mount.nfs Kdump: loaded
               Tainted: G        W         5.16.0-rc5.2585cf9d #1
[  868.573273] Hardware name: QEMU Standard PC (Q35 + ICH9, 2009),
               BIOS 1.14.0-4.fc34 04/01/2014
[  868.574478] Call Trace:
[  868.574844]  <TASK>
[  868.575156]  dump_stack_lvl+0x34/0x44
[  868.575692]  __might_resched.cold+0xd6/0x10f
[  868.576308]  slab_pre_alloc_hook.constprop.0+0x89/0xf0
[  868.577046]  __kmalloc_track_caller+0x72/0x420
[  868.577684]  ? security_context_to_sid_core+0x48/0x2b0
[  868.578569]  kmemdup_nul+0x22/0x50
[  868.579108]  security_context_to_sid_core+0x48/0x2b0
[  868.579854]  ? _nfs4_proc_pathconf+0xff/0x110 [nfsv4]
[  868.580742]  ? nfs_reconfigure+0x80/0x80 [nfs]
[  868.581355]  security_context_str_to_sid+0x36/0x40
[  868.581960]  selinux_sb_mnt_opts_compat+0xb5/0x1e0
[  868.582550]  ? nfs_reconfigure+0x80/0x80 [nfs]
[  868.583098]  security_sb_mnt_opts_compat+0x2a/0x40
[  868.583676]  nfs_compare_super+0x113/0x220 [nfs]
[  868.584249]  ? nfs_try_mount_request+0x210/0x210 [nfs]
[  868.584879]  sget_fc+0xb5/0x2f0
[  868.585267]  nfs_get_tree_common+0x91/0x4a0 [nfs]
[  868.585834]  vfs_get_tree+0x25/0xb0
[  868.586241]  fc_mount+0xe/0x30
[  868.586605]  do_nfs4_mount+0x130/0x380 [nfsv4]
[  868.587160]  nfs4_try_get_tree+0x47/0xb0 [nfsv4]
[  868.587724]  vfs_get_tree+0x25/0xb0
[  868.588193]  do_new_mount+0x176/0x310
[  868.588782]  __x64_sys_mount+0x103/0x140
[  868.589388]  do_syscall_64+0x3b/0x90
[  868.589935]  entry_SYSCALL_64_after_hwframe+0x44/0xae
[  868.590699] RIP: 0033:0x7f2b371c6c4e
[  868.591239] Code: 48 8b 0d dd 71 0e 00 f7 d8 64 89 01 48 83 c8 ff c3 66 2e
                     0f 1f 84 00 00 00 00 00 90 f3 0f 1e fa 49 89 ca b8 a5 00
                     00 00 0f 05 <48> 3d 01 f0 ff ff 73 01 c3 48 8b 0d aa 71
                     0e 00 f7 d8 64 89 01 48
[  868.593810] RSP: 002b:00007ffc83775d88 EFLAGS: 00000246
               ORIG_RAX: 00000000000000a5
[  868.594691] RAX: ffffffffffffffda RBX: 00007ffc83775f10 RCX: 00007f2b371c6c4e
[  868.595504] RDX: 0000555d517247a0 RSI: 0000555d51724700 RDI: 0000555d51724540
[  868.596317] RBP: 00007ffc83775f10 R08: 0000555d51726890 R09: 0000555d51726890
[  868.597162] R10: 0000000000000000 R11: 0000000000000246 R12: 0000555d51726890
[  868.598005] R13: 0000000000000003 R14: 0000555d517246e0 R15: 0000555d511ac925
[  868.598826]  </TASK>

Cc: stable@vger.kernel.org
Fixes: 69c4a42d ("lsm,selinux: add new hook to compare new mount to an existing mount")
Signed-off-by: Scott Mayhew <smayhew@redhat.com>
[PM: cleanup/line-wrap the backtrace]
Signed-off-by: Paul Moore <paul@paul-moore.com>