fix wrong malloc size for ignore_error buffer

fix wrong malloc size for ignore_error buffer

This is supposed to be allocating the initial buffer which consists
of 4 integers to store errno values to ignore.
Also explicitly zero clear the buffer.

Signed-off-by: Tomohiro Kusumi <tkusumi@tuxera.com>
Signed-off-by: Jens Axboe <axboe@fb.com>