The variable declaration in this function predates the merge of the
nrext64 (aka 64-bit extent counters) feature, which means that the
variable declaration type is insufficient to avoid an integer overflow.
Fix that by redeclaring the variable to be xfs_extnum_t.

Coverity-id: 1630958
Fixes: 8f71bede ("xfs: repair inode fork block mapping data structures")
Signed-off-by: Darrick J. Wong <djwong@kernel.org>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Carlos Maiolino <cem@kernel.org>