raid10: check bio in r10buf_pool_free to void NULL pointer dereference

For recovery case, r10buf_pool_alloc only allocates 2 bios,
so we can't access more than 2 bios in r10buf_pool_free.
Otherwise, we can see NULL pointer dereference as follows:

[   98.347009] BUG: unable to handle kernel NULL pointer dereference
at 0000000000000050
[   98.355783] IP: r10buf_pool_free+0x38/0xe0 [raid10]
[...]
[   98.543734] CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
[   98.550161] CR2: 0000000000000050 CR3: 000000089500a001 CR4: 00000000001606f0
[   98.558145] Call Trace:
[   98.560881]  <IRQ>
[   98.563136]  put_buf+0x19/0x20 [raid10]
[   98.567426]  end_sync_request+0x6b/0x70 [raid10]
[   98.572591]  end_sync_write+0x9b/0x160 [raid10]
[   98.577662]  blk_update_request+0x78/0x2c0
[   98.582254]  scsi_end_request+0x2c/0x1e0 [scsi_mod]
[   98.587719]  scsi_io_completion+0x22f/0x610 [scsi_mod]
[   98.593472]  blk_done_softirq+0x8e/0xc0
[   98.597767]  __do_softirq+0xde/0x2b3
[   98.601770]  irq_exit+0xae/0xb0
[   98.605285]  do_IRQ+0x81/0xd0
[   98.608606]  common_interrupt+0x7d/0x7d
[   98.612898]  </IRQ>

So we need to check the bio is valid or not before the bio is
used in r10buf_pool_free. Another workable way is to free 2 bios
for recovery case just like r10buf_pool_alloc.

Fixes: f0250618

 ("md: raid10: don't use bio's vec table to manage resync pages")
Reported-by: Alexis Castilla <pencerval@gmail.com>
Tested-by: Alexis Castilla <pencerval@gmail.com>
Signed-off-by: Guoqing Jiang <gqjiang@suse.com>
Signed-off-by: Shaohua Li <shli@fb.com>