ext4: allow ZERO_RANGE on encrypted files

ext4: allow ZERO_RANGE on encrypted files

When ext4 encryption support was first added, ZERO_RANGE was disallowed,
supposedly because test failures (e.g. ext4/001) were seen when enabling
it, and at the time there wasn't enough time/interest to debug it.

However, there's actually no reason why ZERO_RANGE can't work on
encrypted files.  And it fact it *does* work now.  Whole blocks in the
zeroed range are converted to unwritten extents, as usual; encryption
makes no difference for that part.  Partial blocks are zeroed in the
pagecache and then ->writepages() encrypts those blocks as usual.
ext4_block_zero_page_range() handles reading and decrypting the block if
needed before actually doing the pagecache write.

Also, f2fs has always supported ZERO_RANGE on encrypted files.

As far as I can tell, the reason that ext4/001 was failing in v4.1 was
actually because of one of the bugs fixed by commit 36086d43f657 ("ext4
crypto: fix bugs in ext4_encrypted_zeroout()").  The bug made
ext4_encrypted_zeroout() always return a positive value, which caused
unwritten extents in encrypted files to sometimes not be marked as
initialized after being written to.  This bug was not actually in
ZERO_RANGE; it just happened to trigger during the extents manipulation
done in ext4/001 (and probably other tests too).

So, let's enable ZERO_RANGE on encrypted files on ext4.

Tested with:
	gce-xfstests -c ext4/encrypt -g auto
	gce-xfstests -c ext4/encrypt_1k -g auto

Got the same set of test failures both with and without this patch.
But with this patch 6 fewer tests are skipped: ext4/001, generic/008,
generic/009, generic/033, generic/096, and generic/511.

Signed-off-by: Eric Biggers <ebiggers@google.com>
Link: https://lore.kernel.org/r/20191226154216.4808-1-ebiggers@kernel.org
Signed-off-by: Theodore Ts'o <tytso@mit.edu>