pf_insert: Return matching node on insert

pf_insert: Return matching node on insert

When a matching entry is found from the rb_tree, the insert function
should return the found entry from the tree to the caller. This is
also explicitly documented in the code. However, the code incorrectly
returned NULL instead.

Fix the code to return the match as it should do. Without this fix
identical pageframe entries are never located.

Signed-off-by: Timo Kokkonen <timo.t.kokkonen@iki.fi>