LU-8465 ext2fs: RO open for bitmap loading

LU-8465 ext2fs: RO open for bitmap loading

See following errors:
 e2fsck -fnm 12 /dev/loop0

Multiple threads triggered to read bitmaps
e2fsck: Operation not permitted while retrying to
read bitmaps for 35df4c2c-MDT0000

snapshot device is Readonly, open it with RW will return
EPERM, since parallel bitmap loading is Readonly operation,
use readonly open is ok.

Fixes: 0e888c3cca ("LU-8465 ext2fs: parallel bitmap loading")
DDN-bug-id: EX-2665
Change-Id: I3e5fc1f9360d714f8223b5628aab871b0ac25298
Signed-off-by: Wang Shilong <wshilong@ddn.com>
Reviewed-on: https://review.whamcloud.com/43129
Tested-by: Maloo <maloo@whamcloud.com>
Reviewed-by: Andreas Dilger <adilger@whamcloud.com>