fscrypt: Use correct index in decrypt path.

Actually use the fs-provided index instead of always using page->index
which is only set for page-cache pages.

Fixes: 9c4bb8a3 ("fscrypt: Let fs select encryption index/tweak")

Signed-off-by: David Gstir <david@sigma-star.at>
Signed-off-by: Theodore Ts'o <tytso@mit.edu>