svcrpc: don't hold sv_lock over svc_xprt_put()

svc_xprt_put() can call tcp_close(), which can sleep, so we shouldn't be
holding this lock.

In fact, only the xpt_list removal and the sv_tmpcnt decrement should
need the sv_lock here.

Reported-by: Mi Jinlong <mijinlong@cn.fujitsu.com>
Signed-off-by: J. Bruce Fields <bfields@citi.umich.edu>