asm-generic: fix a compilation warning

Fix this compilation warning on x86 by making flush_cache_vmap() inline.

  lib/ioremap.c: In function 'ioremap_page_range':
  lib/ioremap.c:214:16: warning: variable 'start' set but not used [-Wunused-but-set-variable]
    unsigned long start;
                  ^~~~~

While at it, convert all other similar functions to inline for
consistency.

Link: http://lkml.kernel.org/r/1562594592-15228-1-git-send-email-cai@lca.pw
Signed-off-by: Qian Cai <cai@lca.pw>
Reviewed-by: Andrew Morton <akpm@linux-foundation.org>
Cc: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>