xfs: Use kvcalloc() instead of kvzalloc()

Use 2-factor argument multiplication form kvcalloc() instead of
kvzalloc().

Link: https://github.com/KSPP/linux/issues/162


Signed-off-by: Gustavo A. R. Silva <gustavoars@kernel.org>
Reviewed-by: Darrick J. Wong <djwong@kernel.org>
Signed-off-by: Darrick J. Wong <djwong@kernel.org>