gfs2: fallocate_chunk: Always initialize struct iomap

In fallocate_chunk, always initialize the iomap before calling
gfs2_iomap_get_alloc: future changes could otherwise cause things like
iomap.flags to leak across calls.

Signed-off-by: Andreas Gruenbacher <agruenba@redhat.com>
Reviewed-by: Bob Peterson <rpeterso@redhat.com>