fs/jfs: Limit the extents that getblk() can consider

fs/jfs: Limit the extents that getblk() can consider

getblk() implicitly trusts that treehead->count is an accurate count of
the number of extents. However, that value is read from disk and is not
trustworthy, leading to OOB reads and crashes. I am not sure to what
extent the data read from OOB can influence subsequent program execution.

Require callers to pass in the maximum number of extents for which
they have storage.

Signed-off-by: Daniel Axtens <dja@axtens.net>
Reviewed-by: Daniel Kiper <daniel.kiper@oracle.com>