x86: don't use REP_GOOD or ERMS for user memory clearing

The modern target to use is FSRS (Fast Short REP STOS), and the other
cases should only be used for bigger areas (ie mainly things like page
clearing).

Note! This changes the conditional for the inlining from FSRM ("fast
short rep movs") to FSRS ("fast short rep stos").

We'll have a separate fixup for AMD microarchitectures that have a good
'rep stosb' yet do not set the new Intel-specific FSRS bit (because FSRM
was there first).

Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>