The mutexes swap_slots_cache_mutex and swap_slots_cache_enable_mutex are
local to the source and do not need to be in global scope, so make them
static.

Cleans up sparse warnings:
  symbol 'swap_slots_cache_mutex' was not declared. Should it be static?
  symbol 'swap_slots_cache_enable_mutex' was not declared. Should it be static?

Link: http://lkml.kernel.org/r/20180624182536.4937-1-colin.king@canonical.comSigned-off-by: NColin Ian King <colin.king@canonical.com>
Reviewed-by: NAndrew Morton <akpm@linux-foundation.org>
Signed-off-by: NAndrew Morton <akpm@linux-foundation.org>
Signed-off-by: NLinus Torvalds <torvalds@linux-foundation.org>