Correctly update fs_dsize in growfs(8)

When growing a UFS/FFS filesystem, the size of the summary information
may expand into additional blocks. These blocks must be removed from
fs_dsize which records the number of blocks in the filesystem that can
be used to hold filesystem data.

While here also update the fs_old_dsize and fs_old_size fields for
compatibility with kernels that were compiled before the addition
of UFS2.

Reported by: Edward Tomasz Napiera
MFC after: 1 week