ext4: use s_extent_max_zeroout_kb value as number of kb

Currently when converting extent to initialized, we have to decide
whether to zeroout part/all of the uninitialized extent in order to
avoid extent tree growing rapidly.

The decision is made by comparing the size of the extent with the
configurable value s_extent_max_zeroout_kb which is in kibibytes units.

However when converting it to number of blocks we currently use it as it
was in bytes. This is obviously bug and it will result in ext4 _never_
zeroout extents, but rather always split and convert parts to
initialized while leaving the rest uninitialized in default setting.

Fix this by using s_extent_max_zeroout_kb as kibibytes.

Signed-off-by: Lukas Czerner <lczerner@redhat.com>
Signed-off-by: "Theodore Ts'o" <tytso@mit.edu>
Cc: stable@vger.kernel.org