fscrypt: use READ_ONCE() to access ->i_crypt_info

fscrypt: use READ_ONCE() to access ->i_crypt_info

->i_crypt_info starts out NULL and may later be locklessly set to a
non-NULL value by the cmpxchg() in fscrypt_get_encryption_info().

But ->i_crypt_info is used directly, which technically is incorrect.
It's a data race, and it doesn't include the data dependency barrier
needed to safely dereference the pointer on at least one architecture.

Fix this by using READ_ONCE() instead.  Note: we don't need to use
smp_load_acquire(), since dereferencing the pointer only requires a data
dependency barrier, which is already included in READ_ONCE().  We also
don't need READ_ONCE() in places where ->i_crypt_info is unconditionally
dereferenced, since it must have already been checked.

Also downgrade the cmpxchg() to cmpxchg_release(), since RELEASE
semantics are sufficient on the write side.

Signed-off-by: Eric Biggers <ebiggers@google.com>
Signed-off-by: Theodore Ts'o <tytso@mit.edu>