xfs: fix uninitialized variable access

mainline inclusion
from mainline-v6.2-rc6
commit 60b730a4
category: bugfix
bugzilla: 188220, https://gitee.com/openeuler/kernel/issues/I4KIAO

Reference: https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/commit/?id=60b730a40c43fbcc034970d3e77eb0f25b8cc1cf



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If the end position of a GETFSMAP query overlaps an allocated space and
we're using the free space info to generate fsmap info, the akeys
information gets fed into the fsmap formatter with bad results.
Zero-init the space.

Reported-by:  <syzbot+090ae72d552e6bd93cfe@syzkaller.appspotmail.com>
Signed-off-by: Darrick J. Wong <djwong@kernel.org>
Signed-off-by: Long Li <leo.lilong@huawei.com>