gfs2: Re-enable fallocate for the rindex

Commit 86066914 "gfs2: Don't support
fallocate on jdata files" removed the ability of gfs2_grow to reserve
space at the end of the rindex, which could prevent a second gfs2_grow
from succeeding if the fs is full. Allow fallocate to work on the rindex
once again.

Signed-off-by: Andrew Price <anprice@redhat.com>
Signed-off-by: Bob Peterson <rpeterso@redhat.com>