locking/rwsem-rt: Remove might_sleep() in __up_read()

There's no chance of sleeping here, the reader is giving up the
lock and possibly waking up the writer who is waiting on it.

Reported-by: Chunyu Hu <chuhu@redhat.com>
Signed-off-by: Andrew Halaney <ahalaney@redhat.com>
Signed-off-by: Steven Rostedt (VMware) <rostedt@goodmis.org>