gfs2: Improve mmap write vs. truncate consistency

On filesystems with a block size smaller than PAGE_SIZE, page_mkwrite is
called for each memory-mapped page before that page can be written to.
When such a memory-mapped file is truncated down to size x which is not
a multiple of the page size and then back to a larger size, the page
straddling size x can end up with a partial block mapping.  In that
case, make sure to mark that page read-only so that page_mkwrite will be
called before the page can be written to the next time.

(There is no point in marking the page straddling size x read-only when
truncating down as writing to memory beyond the end of the file will
result in SIGBUS instead of growing the file.)

Fixes xfstests generic/029, generic/030 on filesystems with a block size
smaller than PAGE_SIZE.

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