caif-shm: Bugfixes for caif_shmcore.c

Various bugfixes for caif_shmcore.c:
- fix deadlocks due to improper usage of spin-lock
- add missing spin-lock init
- don't call dev_kfree_skb() with irqs disabled,
  use dev_kfree_skb_irq() instead.
- fix potential skb null pointer de-reference.

Squashed original patches from:
Rabin Vincent <rabin.vincent@stericsson.com>
Durga Prasada Rao BATHINA <durgaprasadarao.b@stericcson.com>
Arun Murthy <arun.murthy@stericsson.com>
Bibek Basu <bibek.basu@stericsson.com>

Signed-off-by: Sjur Brændeland <sjur.brandeland@stericsson.com>
Signed-off-by: David S. Miller <davem@davemloft.net>