nilfs2: fix ignored error code in __nilfs_read_inode()

The __nilfs_read_inode function is ignoring the error code returned
from nilfs_read_inode_common(), and wrongly delivers a success code
(zero) when it escapes from the function in erroneous cases.

This adds the missing error handling.

Signed-off-by: Ryusuke Konishi <konishi.ryusuke@lab.ntt.co.jp>