metadump: catch used extent array overflow

metadump: catch used extent array overflow

An user reported a SIGSEGV when attempting to create a metadump image of
a filesystem.
The reason is because we fail to catch a possible overflow in the
used extents array in process_exinode() which may happen if the extent
count is corrupted.
This leads process_bmbt_reclist() to attempt to index into the array
using the bogus extent count with:

convert_extent(&rp[numrecs - 1], &o, &s, &c, &f);

Fix this by extending the used counter to uint64_t and
checking for the overflow possibility.

Reported-by: hubert . <hubjin657@outlook.com>
Suggested-by: Dave Chinner <david@fromorbit.com>
Signed-off-by: Carlos Maiolino <cmaiolino@redhat.com>
Reviewed-by: Christoph Hellwig <hch@lst.de>