fscrypt: optimize fscrypt_zeroout_range()

Currently fscrypt_zeroout_range() issues and waits on a bio for each
block it writes, which makes it very slow.

Optimize it to write up to 16 pages at a time instead.

Also add a function comment, and improve reliability by allowing the
allocations of the bio and the first ciphertext page to wait on the
corresponding mempools.

Link: https://lore.kernel.org/r/20191226160813.53182-1-ebiggers@kernel.org


Reviewed-by: Theodore Ts'o <tytso@mit.edu>
Signed-off-by: Eric Biggers <ebiggers@google.com>