hfsplus: fix HFSPLUS_I calling convention

HFSPLUS_I doesn't return a pointer to the hfsplus-specific inode
information like all other FOO_I macros, but dereference the pointer in a way
that made it look like a direct struct derefence.  This only works as long
as the HFSPLUS_I macro is used directly and prevents us from keepig a local
hfsplus_inode_info pointer.  Fix the calling convention and introduce a local
hip variable in all functions that use it constantly.

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