ext4 crypto: fix memory leaks in ext4_encrypted_zeroout

ext4_encrypted_zeroout() could end up leaking a bio and bounce page.
Fortunately it's not used much.  While we're fixing things up,
refactor out common code into the static function alloc_bounce_page().

Change-Id: I44023c01de7ec97ad43bfa85cd7d3b97b22ee0c0
Signed-off-by: Theodore Ts'o <tytso@mit.edu>
Signed-off-by: Theodore Ts'o <tytso@google.com>