tracing: Do not reset *pos in set_ftrace_filter

After the filtered functions are read, the probed functions are read
from the hash in set_ftrace_filter. When the hashed probed functions
are read, the *pos passed in is reset. Instead of modifying the pos
given to the read function, just record the pos where the filtered
functions ended and subtract from that.

Signed-off-by: Steven Rostedt <rostedt@goodmis.org>