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/[email protected]
Fixes: 9ad9718bfa41 ("mm/mremap: calculate extent in one place")
Signed-off-by: Arnd Bergmann <[email protected]>
Tested-by: Nick Desaulniers <[email protected]>
Reviewed-by: Nathan Chancellor <[email protected]>
Tested-by: Sedat Dilek <[email protected]>
Cc: Kirill A. Shutemov" <[email protected]>
Cc: Wei Yang <[email protected]>
Cc: Vlastimil Babka <[email protected]>
Cc: Dmitry Safonov <[email protected]>
Cc: Brian Geffon <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>
Signed-off-by: Linus Torvalds <[email protected]>