fscrypt: use smp_load_acquire() for ->s_master_keys

fscrypt: use smp_load_acquire() for ->s_master_keys

Normally smp_store_release() or cmpxchg_release() is paired with
smp_load_acquire().  Sometimes smp_load_acquire() can be replaced with
the more lightweight READ_ONCE().  However, for this to be safe, all the
published memory must only be accessed in a way that involves the
pointer itself.  This may not be the case if allocating the object also
involves initializing a static or global variable, for example.

super_block::s_master_keys is a keyring, which is internal to and is
allocated by the keyrings subsystem.  By using READ_ONCE() for it, we're
relying on internal implementation details of the keyrings subsystem.

Remove this fragile assumption by using smp_load_acquire() instead.

(Note: I haven't seen any real-world problems here.  This change is just
fixing the code to be guaranteed correct and less fragile.)

Fixes: 22d94f493bfb ("fscrypt: add FS_IOC_ADD_ENCRYPTION_KEY ioctl")
Link: https://lore.kernel.org/r/20200721225920.114347-4-ebiggers@kernel.org
Signed-off-by: Eric Biggers <ebiggers@google.com>