xfs: complain if we don't get nextents bmap records

When reading into memory all extents of a btree-format inode fork,
complain if the number of extents we find is not the same as the number
of extents reported in the inode core.  This is needed to stop an IO
action from accessing the garbage areas of the in-core fork.

[dchinner: removed redundant assert]

Signed-off-by: Darrick J. Wong <darrick.wong@oracle.com>
Reviewed-by: Dave Chinner <dchinner@redhat.com>
Signed-off-by: Dave Chinner <david@fromorbit.com>