sched.h: always_inline alloc_tag_{save|restore} to fix modpost warnings

Mark alloc_tag_{save|restore} as always_inline to fix the following
modpost warnings:

WARNING: modpost: vmlinux: section mismatch in reference: alloc_tag_save+0x1c (section: .text.unlikely) -> initcall_level_names (section: .init.data)
WARNING: modpost: vmlinux: section mismatch in reference: alloc_tag_restore+0x3c (section: .text.unlikely) -> initcall_level_names (section: .init.data)

The warnings happen when these functions are called from an __init
function and they don't get inlined (remain in the .text section) while
the value returned by get_current() points into .init.data section. 
Assuming get_current() always returns a valid address, this situation can
happen only during init stage and accessing .init.data from .text section
during that stage should pose no issues.

Link: https://lkml.kernel.org/r/20240704132506.1011978-1-surenb@google.com
Fixes: 22d407b1

 ("lib: add allocation tagging support for memory allocation profiling")
Signed-off-by: Suren Baghdasaryan <surenb@google.com>
Reported-by: kernel test robot <lkp@intel.com>
Closes: https://lore.kernel.org/oe-kbuild-all/202407032306.gi9nZsBi-lkp@intel.com/
Cc: Kent Overstreet <kent.overstreet@linux.dev>
Cc: Chris Zankel <chris@zankel.net>
Cc: Ingo Molnar <mingo@redhat.com>
Cc: Juri Lelli <juri.lelli@redhat.com>
Cc: Max Filippov <jcmvbkbc@gmail.com>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Vincent Guittot <vincent.guittot@linaro.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>