hwrng: core - sleep interruptible in read

hwrng kthread can be waiting via hwrng_fillfn for some data from a rng
like virtio-rng:
hwrng           D ffff880093e17798     0   382      2 0x00000000
...
Call Trace:
 [<ffffffff817339c6>] wait_for_completion_killable+0x96/0x210
 [<ffffffffa00aa1b7>] virtio_read+0x57/0xf0 [virtio_rng]
 [<ffffffff814f4a35>] hwrng_fillfn+0x75/0x130
 [<ffffffff810aa243>] kthread+0xf3/0x110

And when some user program tries to read the /dev node in this state,
we get:
rngd            D ffff880093e17798     0   762      1 0x00000004
...
Call Trace:
 [<ffffffff817351ac>] mutex_lock_nested+0x15c/0x3e0
 [<ffffffff814f478e>] rng_dev_read+0x6e/0x240
 [<ffffffff81231958>] __vfs_read+0x28/0xe0
 [<ffffffff81232393>] vfs_read+0x83/0x130

And this is indeed unkillable. So use mutex_lock_interruptible
instead of mutex_lock in rng_dev_read and exit immediatelly when
interrupted. And possibly return already read data, if any (as POSIX
allows).

v2: use ERESTARTSYS instead of EINTR

Signed-off-by: Jiri Slaby <jslaby@suse.cz>
Cc: Matt Mackall <mpm@selenic.com>
Cc: Herbert Xu <herbert@gondor.apana.org.au>
Cc: <linux-crypto@vger.kernel.org>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>