hfsplus: add error checking for hfs_find_init()

hfs_find_init() may fail with ENOMEM, but there are places, where
the returned value is not checked. The consequences can be very
unpleasant, e.g. kfree uninitialized pointer and
inappropriate mutex unlocking.

The patch adds checks for errors in hfs_find_init().

Found by Linux Driver Verification project (linuxtesting.org).

Signed-off-by: Alexey Khoroshilov <khoroshilov@ispras.ru>
Signed-off-by: Christoph Hellwig <hch@lst.de>