btrfs-progs: Return more meaningful value for btrfs_read_deve_super

btrfs-progs: Return more meaningful value for btrfs_read_deve_super

btrfs_read_dev_super() only returns 0 or -1, which doesn't really help,
caller won't know if it's caused by bad superblock or superblock out of
range.

Return -errno if pread64() return -1, and return -EOF if none or part of
the super is read out, and return what check_super() returned.

So caller can get -EIO to catch real corrupted super blocks.

Signed-off-by: Qu Wenruo <quwenruo@cn.fujitsu.com>
Signed-off-by: David Sterba <dsterba@suse.com>