rxrpc: Fix _usecs_to_jiffies() by using usecs_to_jiffies()

Directly using _usecs_to_jiffies() might be unsafe, so it's
better to use usecs_to_jiffies() instead.
Because we can see that the result of _usecs_to_jiffies()
could be larger than MAX_JIFFY_OFFSET values without the
check of the input.

Fixes: c410bf01

 ("Fix the excessive initial retransmission timeout")
Signed-off-by: Jiasheng Jiang <jiasheng@iscas.ac.cn>
Signed-off-by: David S. Miller <davem@davemloft.net>