LU-8465 e2fsck: fix build for make rpm

LU-8465 e2fsck: fix build for make rpm

Two problems addressed by this patch:

1)link e2fsck with -lpthread properly to make rpm pass.
2)a bounch of ext2fs_get_avg_group() unused warning messages are
annoying, move it to rw_bitmaps.c to make gcc happy.

Signed-off-by: Wang Shilong <wshilong@ddn.com>
Change-Id: Ia7d372b7b5aadcbf5d94916f6f67363a2a9f0bfa
Reviewed-on: https://review.whamcloud.com/40060
Tested-by: jenkins <devops@whamcloud.com>
Tested-by: Maloo <maloo@whamcloud.com>
Reviewed-by: Andreas Dilger <adilger@whamcloud.com>