ext4: zero out small extents when writing to prealloc area.

If the preallocated area is small zero out the full extent
instead of splitting them. This should avoid the "write
every alternate block" problem that could grow the number
of extents dramatically.

Signed-off-by: Aneesh Kumar K.V <aneesh.kumar@linux.vnet.ibm.com>
Signed-off-by: Mingming Cao <cmm@us.ibm.com>
Signed-off-by: "Theodore Ts'o" <tytso@mit.edu>