set_memory: add __must_check to generic stubs

Following query shows that architectures that don't provide
asm/set_memory.h don't use set_memory_...() functions.

  $ git grep set_memory_ alpha arc csky hexagon loongarch m68k microblaze mips nios2 openrisc parisc sh sparc um xtensa

Following query shows that all core users of set_memory_...()
functions always take returned value into account:

  $ git grep -w -e set_memory_ro -e set_memory_rw -e set_memory_x -e set_memory_nx -e set_memory_rox `find . -maxdepth 1 -type d | grep -v arch | grep /`

set_memory_...() functions can fail, leaving the memory attributes
unchanged. Make sure all callers check the returned code.

Link: https://github.com/KSPP/linux/issues/7
Link: https://lkml.kernel.org/r/6a89ffc69666de84721216947c6b6c7dcca39d7d.1725723347.git.christophe.leroy@csgroup.eu


Signed-off-by: Christophe Leroy <christophe.leroy@csgroup.eu>
Cc: Arnd Bergmann <arnd@arndb.de>
Cc: Kees Cook <kees@kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>