LU-14432 ext2fs: fix ext2fs_get_avg_group() warning

LU-14432 ext2fs: fix ext2fs_get_avg_group() warning

Move ext2fs_get_avg_group() into e2fsck/pass1.c instead of the
ext2fs.h header, since it is currently only used in that one
function.  It is too large to be a static inline function, and
it is currently causing many compile warnings because it is
not marked with "_INLINE_".

Signed-off-by: Andreas Dilger <adilger@whamcloud.com>
Change-Id: Ie948bc10f44dccf91558099a5217ea1fb5ce7057
Reviewed-on: https://review.whamcloud.com/43881
Tested-by: jenkins <devops@whamcloud.com>
Reviewed-by: Wang Shilong <wshilong@whamcloud.com>
Tested-by: Maloo <maloo@whamcloud.com>