Turn gfs2_block_truncate_page into gfs2_block_zero_range

Turn gfs2_block_truncate_page into a function that zeroes a range within
a block rather than only the end of a block.  This will be used for
cleaning the end of the first partial block and the start of the last
partial block when punching a hole in a file.

Signed-off-by: Andreas Gruenbacher <agruenba@redhat.com>
Signed-off-by: Bob Peterson <rpeterso@redhat.com>