f2fs: fix return values from validate superblock

validate super block is not returning with proper values.
When failure from sb_bread it should reflect there is an EIO otherwise
it should return of EINVAL.
Returning, '1' is not conveying proper message as the return type.

Signed-off-by: Namjae Jeon <namjae.jeon@samsung.com>
Signed-off-by: Amit Sahrawat <a.sahrawat@samsung.com>
Signed-off-by: Jaegeuk Kim <jaegeuk.kim@samsung.com>