hfsplus: fix failed mount handling

Currently the error handling in hfsplus_fill_super is a mess, and can
lead to accessing fields in the superblock that haven't been even set
up yet.  Fix this by making sure we do not set up sb->s_root until we
have the mount fully set up, and before that do proper step by step
unwinding instead of using hfsplus_put_super as a big hammer.

Reported-by: Dan Williams <dcbw@redhat.com>
Signed-off-by: Christoph Hellwig <hch@tuxera.com>