gfs2_grow: Put back the definition of FALLOC_FL_KEEP_SIZE
    
    An #include <linux/falloc.h> was removed in commit 14193bb. Although
    fallocate(2) support was added to glibc 2.10, the FALLOC_FL_*
    definitions were not added until 2.18. This means that gfs2-utils failed
    to build on RHEL7 which has glibc 2.17. Add back the FALLOC_FL_KEEP_SIZE
    definition in main_grow.c to fix that.
    
    Signed-off-by: Andrew Price <anprice@redhat.com>