fs/hfsplus: Set a grub_errno if mount fails

fs/hfsplus: Set a grub_errno if mount fails

It was possible for mount to fail but not set grub_errno. This led to
a possible double decrement of the module reference count if the NULL
page was mapped.

Fixing in general as a similar bug was fixed in commit 61b13c187
(fs/hfsplus: Set grub_errno to prevent NULL pointer access) and there
are likely more variants around.

Fixes: CVE-2024-45783
Reported-by: B Horn <b@horn.uk>
Signed-off-by: B Horn <b@horn.uk>
Reviewed-by: Daniel Kiper <daniel.kiper@oracle.com>