ext4 crypto: use per-inode tfm structure

As suggested by Herbert Xu, we shouldn't allocate a new tfm each time
we read or write a page.  Instead we can use a single tfm hanging off
the inode's crypt_info structure for all of our encryption needs for
that inode, since the tfm can be used by multiple crypto requests in
parallel.

Also use cmpxchg() to avoid races that could result in crypt_info
structure getting doubly allocated or doubly freed.

Change-Id: I4ae5c07d0e5d99ec1e26eeb49d833c4a284d9a5f
Signed-off-by: Theodore Ts'o <tytso@mit.edu>
Signed-off-by: Theodore Ts'o <tytso@google.com>