hfs: fix BUG on bnode parent update

hfs_brec_update_parent() may hit BUG_ON() if the first record of both a
leaf node and its parent are changed, and if this forces the parent to
be split.  It is not possible for this to happen on a valid hfs
filesystem because the index nodes have fixed length keys.

For reasons I ignore, the hfs module does have support for a number of
hfsplus features.  A corrupt btree header may report variable length
keys and trigger this BUG, so it's better to fix it.

Link: http://lkml.kernel.org/r/cf9b02d57f806217a2b1bf5db8c3e39730d8f603.1535682463.git.ernesto.mnd.fernandez@gmail.com
Signed-off-by: Ernesto A. Fernández <ernesto.mnd.fernandez@gmail.com>
Reviewed-by: Andrew Morton <akpm@linux-foundation.org>
Cc: Christoph Hellwig <hch@infradead.org>
Cc: Viacheslav Dubeyko <slava@dubeyko.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>