ltp: add support for FALLOC_FL_WRITE_ZEROES to fsx and fsstress

ltp: add support for FALLOC_FL_WRITE_ZEROES to fsx and fsstress

Add FALLOC_FL_WRITE_ZEROES support to both fsx and fsstress.
FALLOC_FL_WRITE_ZEROES flag was added in the kernel as a part of Zhang
Yi's series [1].

FALLOC_FL_KEEP_SIZE is not supported when FALLOC_FL_WRITE_ZEROES is
used.

[1] https://lore.kernel.org/linux-fsdevel/20250619111806.3546162-1-yi.zhang@huaweicloud.com/

Signed-off-by: Pankaj Raghav <p.raghav@samsung.com>
Reviewed-by: Darrick J. Wong <djwong@kernel.org>
Signed-off-by: Zorro Lang <zlang@kernel.org>