net: split kmalloc_reserve() to allow inlining

net: split kmalloc_reserve() to allow inlining

kmalloc_reserve() is too big to be inlined.

Put the slow path in a new out-of-line function : kmalloc_pfmemalloc()

Then let kmalloc_reserve() set skb->pfmemalloc only when/if
the slow path is taken.

This makes __alloc_skb() faster :

- kmalloc_reserve() is now automatically inlined by both gcc and clang.
- No more expensive RMW (skb->pfmemalloc = pfmemalloc).
- No more expensive stack canary (for CONFIG_STACKPROTECTOR_STRONG=y).
- Removal of two prefetches that were coming too late for modern cpus.

Text size increase is quite small compared to the cpu savings (~0.7 %)

$ size net/core/skbuff.clang.before.o net/core/skbuff.clang.after.o
   text	   data	    bss	    dec	    hex	filename
  72507	   5897	      0	  78404	  13244	net/core/skbuff.clang.before.o
  72681	   5897	      0	  78578	  132f2	net/core/skbuff.clang.after.o

Signed-off-by: Eric Dumazet <edumazet@google.com>
Link: https://patch.msgid.link/20260116041359.181104-1-edumazet@google.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>