isofs: Do not return EACCES for unknown filesystems

commit a2ed0b391dd9c3ef1d64c7c3e370f4a5ffcd324a upstream.

When isofs_mount() is called to mount a device read-write, it returns
EACCES even before it checks that the device actually contains an isofs
filesystem. This may confuse mount(8) which then tries to mount all
subsequent filesystem types in read-only mode.

Fix the problem by returning EACCES only once we verify that the device
indeed contains an iso9660 filesystem.

Fixes: 17b7f7cf58926844e1dd40f5eb5348d481deca6a
Reported-by: Kent Overstreet <kent.overstreet@gmail.com>
Reported-by: Karel Zak <kzak@redhat.com>
Signed-off-by: Jan Kara <jack@suse.cz>
Signed-off-by: Willy Tarreau <w@1wt.eu>