BACKPORT: bpf: Add file mode configuration into bpf maps

Introduce the map read/write flags to the eBPF syscalls that returns the
map fd. The flags is used to set up the file mode when construct a new
file descriptor for bpf maps. To not break the backward capability, the
f_flags is set to O_RDWR if the flag passed by syscall is 0. Otherwise
it should be O_RDONLY or O_WRONLY. When the userspace want to modify or
read the map content, it will check the file mode to see if it is
allowed to make the change.

Signed-off-by: Chenbo Feng <fengc@google.com>
Acked-by: Alexei Starovoitov <ast@kernel.org>
Acked-by: Daniel Borkmann <daniel@iogearbox.net>
Signed-off-by: David S. Miller <davem@davemloft.net>

Deleted the file mode configuration code in unsupported map type and
removed the file mode check in non-existing helper functions.
(cherry-pick from net-next: 6e71b04a82248ccf13a94b85cbc674a9fefe53f5)
Bug: 30950746

Change-Id: Icfad20f1abb77f91068d244fb0d87fa40824dd1b