fscrypt: new helper function - fscrypt_file_open()

Add a helper function which prepares to open a regular file which may be
encrypted.  It handles setting up the file's encryption key, then
checking that the file's encryption policy matches that of its parent
directory (if the parent directory is encrypted).  It may be set as the
->open() method or it can be called from another ->open() method.

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