[GFS2] Fix cast from unsigned int to s64

This fixes bz 444829 where allocating a new block caused gfs2 file systems to
report 0 bytes used in df. It was caused by a broken cast from an unsigned int
in gfs2_block_alloc() to a negative s64 in gfs2_statfs_change(). This patch
casts the unsigned int to an s64 before the unary minus is applied.

Signed-off-by: Andrew Price <andy@andrewprice.me.uk>
Signed-off-by: Steven Whitehouse <swhiteho@redhat.com>