x86/mm: Fix -Wmissing-prototypes warnings for arch/x86/mm/init.c

Fix -Wmissing-prototypes warnings:

  arch/x86/mm/init.c:81:6:
  warning: no previous prototype for ‘x86_has_pat_wp’ [-Wmissing-prototypes]
  bool x86_has_pat_wp(void)

  arch/x86/mm/init.c:86:22:
  warning: no previous prototype for ‘pgprot2cachemode’ [-Wmissing-prototypes]
  enum page_cache_mode pgprot2cachemode(pgprot_t pgprot)

by including the respective header containing prototypes. Also fix:

  arch/x86/mm/init.c:893:13:
  warning: no previous prototype for ‘mem_encrypt_free_decrypted_mem’ [-Wmissing-prototypes]
  void __weak mem_encrypt_free_decrypted_mem(void) { }

by making it static inline for the !CONFIG_AMD_MEM_ENCRYPT case. This
warning happens when CONFIG_AMD_MEM_ENCRYPT is not enabled (defconfig
for example):

  ./arch/x86/include/asm/mem_encrypt.h:80:27:
  warning: inline function ‘mem_encrypt_free_decrypted_mem’ declared weak [-Wattributes]
  static inline void __weak mem_encrypt_free_decrypted_mem(void) { }
                          ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

It's ok to convert to static inline because the function is used only in
x86. Is not shared with other architectures so drop the __weak too.

 [ bp: Massage and adjust __weak comments while at it. ]

Signed-off-by: Benjamin Thiel <b.thiel@posteo.de>
Signed-off-by: Borislav Petkov <bp@suse.de>
Link: https://lkml.kernel.org/r/20200606122629.2720-1-b.thiel@posteo.de