fs/befs/linuxvfs.c: check superblock before dump operation

befs_dump_super_block was called between befs_load_sb and befs_check_sb.
It has been reported to crash (5/900) with null block testing.

This patch loads, checks and only dump superblock if it's a valid one
then brelse bh.

(befs_dump_super_block uses disk_sb (bh->b_data) so it seems we need to
call it before brelse(bh) but I don't know why befs_check_sb was called
after brelse.  Another thing I don't understand is why this problem
appears now).

Signed-off-by: Fabian Frederick <fabf@skynet.be>
Reported-by: Fengguang Wu <fengguang.wu@intel.com>
Cc: Joe Perches <joe@perches.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>