hfsplus: create correct initial catalog entries for device files

Make sure the initial insertation of the catalog entry already contains
the device number by calling init_special_inode early and setting writing
out the dev field of the on-disk permission structure.  The latter is
facilitated by sharing the almost identical hfsplus_set_perms helpers
between initial catalog entry creating and ->write_inode.

Unless we crashed just after mknod this bug was harmless as the inode
is marked dirty at the end of hfsplus_mknod, and hfsplus_write_inode
will update the catalog entry to contain the correct value.

Signed-off-by: Christoph Hellwig <hch@tuxera.com>