mm/mremap: fix BUILD_BUG_ON() error in get_extent

mm/mremap: fix BUILD_BUG_ON() error in get_extent

clang can't evaluate this function argument at compile time when the
function is not inlined, which leads to a link time failure:

  ld.lld: error: undefined symbol: __compiletime_assert_414
  >>> referenced by mremap.c
  >>>               mremap.o:(get_extent) in archive mm/built-in.a

Mark the function as __always_inline to avoid it.

Link: https://lkml.kernel.org/r/20201230154104.522605-1-arnd@kernel.org
Fixes: 9ad9718bfa41 ("mm/mremap: calculate extent in one place")
Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Tested-by: Nick Desaulniers <ndesaulniers@google.com>
Reviewed-by: Nathan Chancellor <natechancellor@gmail.com>
Tested-by: Sedat Dilek <sedat.dilek@gmail.com>
Cc: Kirill A. Shutemov" <kirill.shutemov@linux.intel.com>
Cc: Wei Yang <richard.weiyang@linux.alibaba.com>
Cc: Vlastimil Babka <vbabka@suse.cz>
Cc: Dmitry Safonov <0x7f454c46@gmail.com>
Cc: Brian Geffon <bgeffon@google.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>