Merge branch 'introduce BPF_F_PRESERVE_ELEMS'

Song Liu says:

====================
This set introduces BPF_F_PRESERVE_ELEMS to perf event array for better
sharing of perf event. By default, perf event array removes the perf event
when the map fd used to add the event is closed. With BPF_F_PRESERVE_ELEMS
set, however, the perf event will stay in the array until it is removed, or
the map is closed.
---
Changes v3 => v5:
1. Clean up in selftest. (Alexei)

Changes v2 => v3:
1. Move perf_event_fd_array_map_free() to avoid unnecessary forward
   declaration. (Daniel)

Changes v1 => v2:
1. Rename the flag as BPF_F_PRESERVE_ELEMS. (Alexei, Daniel)
2. Simplify the code and selftest. (Daniel, Alexei)
====================

Signed-off-by: Alexei Starovoitov <ast@kernel.org>