GFS2: Change maxlen variables to size_t

This patch changes some variables (especially maxlen in function
gfs2_block_map) from unsigned int to size_t. We need 64-bit arithmetic
for very large files (e.g. 1PB) where the variables otherwise get
shifted to all 0's.

Signed-off-by: Bob Peterson <rpeterso@redhat.com>
Signed-off-by: Steven Whitehouse <swhiteho@redhat.com>