GFS2: Fix typo in gfs_page_mkwrite()

There is a typo in gfs2_page_mkwrite()

gfs2_write_alloc_required() expects pos to be the offset in bytes. However,
instead of the page index being shifted by by PAGE_CACHE_SHIFT, it was shifted
by (PAGE_CACHE_SIZE - inode->i_blkbits).  This patch simply shifts the page
index by the proper amount.

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