GFS2: Streamline alloc calculations for writes

This patch removes some unused code, and make the calculation
of the number of blocks required conditional in order to reduce
the number of times this (potentially expensive) calculation
is done.

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

diff --cc fs/gfs2/bmap.c
Simple merge

diff --cc fs/gfs2/bmap.h
Simple merge

diff --cc fs/gfs2/ops_address.c
Simple merge

diff --cc fs/gfs2/ops_file.c
Simple merge