Merge branch 'bpf-xskmap-perf-improvements'

Björn Töpel says:

====================
This set consists of three patches from Maciej and myself which are
optimizing the XSKMAP lookups.  In the first patch, the sockets are
moved to be stored at the tail of the struct xsk_map. The second
patch, Maciej implements map_gen_lookup() for XSKMAP. The third patch,
introduced in this revision, moves various XSKMAP functions, to permit
the compiler to do more aggressive inlining.

Based on the XDP program from tools/lib/bpf/xsk.c where
bpf_map_lookup_elem() is explicitly called, this work yields a 5%
improvement for xdpsock's rxdrop scenario. The last patch yields 2%
improvement.

Jonathan's Acked-by: for patch 1 and 2 was carried on. Note that the
overflow checks are done in the bpf_map_area_alloc() and
bpf_map_charge_init() functions, which was fixed in commit
ff1c08e1

 ("bpf: Change size to u64 for bpf_map_{area_alloc,
charge_init}()").

  [1] https://patchwork.ozlabs.org/patch/1186170/

v1->v2: * Change size/cost to size_t and use {struct, array}_size
          where appropriate. (Jakub)
v2->v3: * Proper commit message for patch 2.
v3->v4: * Change size_t to u64 to handle 32-bit overflows. (Jakub)
        * Introduced patch 3.
v4->v5: * Use BPF_SIZEOF size, instead of BPF_DW, for correct
          pointer-sized loads. (Daniel)
====================

Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>