fs/hfsplus: Allow reading files created by Mac OS 9

fs/hfsplus: Allow reading files created by Mac OS 9

The "permissions" field of hfsplus files is only used by Mac OS X. This
causes GRUB to skip reading files created by Mac OS 9, since their
file mode is read as unknown. Instead, assume files with zero mode
are regular files.

From Technote 1150:

  The traditional Mac OS implementation of HFS Plus does not use the
  permissions field. Files created by traditional Mac OS have the
  entire field set to 0.

Signed-off-by: Dave Vasilevsky <dave@vasilevsky.ca>
Reviewed-by: Daniel Kiper <daniel.kiper@oracle.com>