Speed up gfs_grow
    
    bz 485451 -  gfs_grow very slow with 1k file system block size
    
    This patch speeds up gfs_grow by doing two main things:
    First, it doesn't close/open/seek for every RG block write.
    Second, it combines writes to the rindex file so that a
    whole page may be allocated and written at a time.  That
    avoids some very slow glock issues, especially when block size
    is less than page size.