lib/string: optimized memmove

lib/string: optimized memmove

When the destination buffer is before the source one, or when the buffers
doesn't overlap, it's safe to use memcpy() instead, which is optimized to
use a bigger data size possible.

This "optimization" only covers a common case.  In future, proper code
which does the same thing as memcpy() does but backwards can be done.

Link: https://lkml.kernel.org/r/20210702123153.14093-3-mcroce@linux.microsoft.com
Signed-off-by: Matteo Croce <mcroce@microsoft.com>
Cc: Christoph Hellwig <hch@infradead.org>
Cc: David Laight <David.Laight@aculab.com>
Cc: Drew Fustini <drew@beagleboard.org>
Cc: Emil Renner Berthing <kernel@esmil.dk>
Cc: Guo Ren <guoren@kernel.org>
Cc: Nick Desaulniers <ndesaulniers@google.com>
Cc: Nick Kossifidis <mick@ics.forth.gr>
Cc: Palmer Dabbelt <palmer@dabbelt.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Stephen Rothwell <sfr@canb.auug.org.au>