src/ext4_resize.c: set errno to 0 before the strtoull call

src/ext4_resize.c: set errno to 0 before the strtoull call

On my test machine, ext4/033 fails even use the non-overflow size.
It reports invalid new size when using strtoull because errno is 1.

As man-pages said "Since  strtoul()  can legitimately return 0 or ULONG_MAX
(ULLONG_MAX for strtoull()) on both success and failure, the calling program
should set errno to 0 before the call, and then determine if an error occurred
by checking whether errno has a nonzero value after the call".

So add a step to set errno to 0 before strtoull call.

Fixes: 92b9c0dedace ("ext4/033: test EXT4_IOC_RESIZE_FS by calling the ioctl directly")
Signed-off-by: Yang Xu <xuyang2018.jy@fujitsu.com>
Reviewed-by: Darrick J. Wong <djwong@kernel.org>
Reviewed-by: Theodore Ts'o <tytso@mit.edu>
Signed-off-by: Eryu Guan <guaneryu@gmail.com>