md/raid10: initialize r10_bio->read_slot before use.

In __make_request() a new r10bio is allocated and passed to
raid10_read_request(). The read_slot member of the bio is not
initialized, and the raid10_read_request() uses it to index an
array. This leads to occasional panics.

Fix by initializing the field to invalid value and checking for
valid value in raid10_read_request().

Cc: stable@vger.kernel.org
Signed-off-by: Kevin Vigor <kvigor@gmail.com>
Signed-off-by: Song Liu <songliubraving@fb.com>