udf: Make udf_read_inode() and udf_iget() return error

Currently __udf_read_inode() wasn't returning anything and we found out
whether we succeeded reading inode by checking whether inode is bad or
not. udf_iget() returned NULL on failure and inode pointer otherwise.
Make these two functions properly propagate errors up the call stack and
use the return value in callers.

Signed-off-by: Jan Kara <jack@suse.cz>