Enhance btrfs_check_super_valid() function by the following points:
1) Restrict sector/node size check
   Not the old max/min valid check, but also check if it's a power of 2.
   So some bogus number like 12K node size won't pass now.

2) Super flag check
   For now, there is still some inconsistency between kernel and
   btrfs-progs super flags.
   And considering btrfs-progs may add new flags for super block, this
   check will only output warning.

3) Better root alignment check
   Now root bytenr is checked against sector size.

4) Move some check into btrfs_check_super_valid().
   Like node size vs leaf size check, and PAGESIZE vs sectorsize check.
   And magic number check.
Reported-by: Vegard Nossum <vegard.nossum@oracle.com>
Signed-off-by: Qu Wenruo <quwenruo@cn.fujitsu.com>
Reviewed-by: David Sterba <dsterba@suse.com>
Signed-off-by: Chris Mason <clm@fb.com>