xfs: fix uninitialized variable access

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>