fscrypt: avoid data race on fscrypt_mode::logged_impl_name

fscrypt: avoid data race on fscrypt_mode::logged_impl_name

The access to logged_impl_name is technically a data race, which tools
like KCSAN could complain about in the future.  See:
https://github.com/google/ktsan/wiki/READ_ONCE-and-WRITE_ONCE

Fix by using xchg(), which also ensures that only one thread does the
logging.

This also required switching from bool to int, to avoid a build error on
the RISC-V architecture which doesn't implement xchg on bytes.

Signed-off-by: Eric Biggers <ebiggers@google.com>