isofs: Fix lseek() to position beyond 4 GB

isofs supports files larger than 4 GB by using multi-extent files.
However an lseek() to a position beyond 4 GB in such a file will
fail with EINVAL, because s_maxbytes in the isofs superblock is
initialized to 2^32-1, and generic_file_llseek() checks against
that value.

I therefore suggest increasing the value of s_maxbytes to have
full support for large files in isofs. With multi-extent files, file
size is only limited by the maximum size of the file system (8 TB),
so this seems a reasonable value for s_maxbytes.

Signed-off-by: Jan Andres <jandres@gmx.net>
Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>