gfs2: page poisoning fix

gfs2: page poisoning fix

Processes can write to the last page of a file using mmap, and when the file
size is not a multiple of the page size, this can be used to write beyond the
end of the file.  This is sometimes referred to as page poisoning, and it is
not a problem in itself because the data beyond eof will be ignored.  However,
we currently fail to clear out any space beyond the end of the file that we
skip over when the file size is increased, so that "poison" can end up getting
exposed.  Fix that.

Fixes xfstest generic/363.

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