crash_dump: fix dm_crypt keys locking and ref leak

crash_dump: fix dm_crypt keys locking and ref leak

crash_load_dm_crypt_keys() reads dm-crypt volume keys from the user
keyring.  It uses user_key_payload_locked() without holding key->sem,
which makes lockdep complain when kexec_file_load() assembles the crash
image:

  =============================
  WARNING: suspicious RCU usage
  -----------------------------
  ./include/keys/user-type.h:53 suspicious rcu_dereference_protected() usage!

  other info that might help us debug this:

  rcu_scheduler_active = 2, debug_locks = 1
  no locks held by kexec/4875.

  stack backtrace:
  Call Trace:
   <TASK>
   dump_stack_lvl+0x5d/0x80
   lockdep_rcu_suspicious.cold+0x4e/0x96
   crash_load_dm_crypt_keys+0x314/0x390
   bzImage64_load+0x116/0x9a0
   ? __lock_acquire+0x464/0x1ba0
   __do_sys_kexec_file_load+0x26a/0x4f0
   do_syscall_64+0xbd/0x430
   entry_SYSCALL_64_after_hwframe+0x77/0x7f

In addition, the key returned by request_key() is never key_put()'d,
leaking a key reference on each load attempt.

Take key->sem while copying the payload and drop the key reference
afterwards.

Link: https://lkml.kernel.org/r/patch.git-2d4d76083a5c.your-ad-here.call-01769426386-ext-2560@work.hours
Fixes: 479e58549b0f ("crash_dump: store dm crypt keys in kdump reserved memory")
Signed-off-by: Vasily Gorbik <gor@linux.ibm.com>
Cc: Baoquan He <bhe@redhat.com>
Cc: Coiby Xu <coxu@redhat.com>
Cc: Dave Young <dyoung@redhat.com>
Cc: Vivek Goyal <vgoyal@redhat.com>
Cc: <stable@vger.kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>