LU-14154 ldiskfs: gcc10 support

gcc10 complains about uninitialized fields:
ldiskfs/extents.c: In function ‘ldiskfs_ext_map_blocks’:
ldiskfs/ldiskfs_extents.h:214:12: error: 'zero_ex2.ee_start_hi' may
be used uninitialized in this function [-Werror=maybe-uninitialized]

Signed-off-by: Alex Zhuravlev <bzzz@whamcloud.com>
Change-Id: I17ef56352194f6ad76bf6f08dab0be3e28d0323c
Reviewed-on: https://review.whamcloud.com/40776
Tested-by: jenkins <devops@whamcloud.com>
Reviewed-by: Andreas Dilger <adilger@whamcloud.com>
Reviewed-by: Yang Sheng <ys@whamcloud.com>
Tested-by: Maloo <maloo@whamcloud.com>