xfs_repair: validate on-disk extent count better

xfs_repair: validate on-disk extent count better

When scanning a btree format inode, we trust the extent count to be
in range.  However, values of the range 2^31 <= cnt < 2^32 are
invalid and can cause problems with signed range checks. This
results in assert failures which validating the extent count such
as:

xfs_repair: dinode.c:768: process_bmbt_reclist_int: Assertion `i < *numrecs' failed.

Validate the extent count is at least within the positive range of a
signed 32 bit integer before using it.

Signed-off-by: Dave Chinner <dchinner@redhat.com>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Mark Tinguely <tinguely@sgi.com>
Signed-off-by: Rich Johnston <rjohnston@sgi.com>