alloc_tag: avoid current->alloc_tag manipulations when profiling is disabled

When memory allocation profiling is disabled there is no need to update
current->alloc_tag and these manipulations add unnecessary overhead.  Fix
the overhead by skipping these extra updates.

I ran comprehensive testing on Pixel 6 on Big, Medium and Little cores:

                 Overhead before fixes            Overhead after fixes
                 slab alloc      page alloc          slab alloc      page alloc
Big               6.21%           5.32%                3.31%          4.93%
Medium            4.51%           5.05%                3.79%          4.39%
Little            7.62%           1.82%                6.68%          1.02%

This is an allocation microbenchmark doing allocations in a tight loop. 
Not a really realistic scenario and useful only to make performance
comparisons.

Link: https://lkml.kernel.org/r/20241226211639.1357704-1-surenb@google.com


Fixes: b951aaff ("mm: enable page allocation tagging")
Signed-off-by: Suren Baghdasaryan <surenb@google.com>
Cc: David Wang <00107082@163.com>
Cc: Kent Overstreet <kent.overstreet@linux.dev>
Cc: Yu Zhao <yuzhao@google.com>
Cc: Zhenhua Huang <quic_zhenhuah@quicinc.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>