nvme-pci: make nvme_pci_npages_prp() __always_inline

nvme-pci: make nvme_pci_npages_prp() __always_inline

The only reason nvme_pci_npages_prp() could be used as a compile-time
known result in BUILD_BUG_ON() is because the compiler was always choosing
to inline the function. Under special circumstances (sanitizer coverage
functions disabled for __init functions on ARCH=um), the compiler decided
to stop inlining it:

   drivers/nvme/host/pci.c: In function 'nvme_init':
   include/linux/compiler_types.h:557:45: error: call to '__compiletime_assert_678' declared with attribute error: BUILD_BUG_ON failed: nvme_pci_npages_prp() > NVME_MAX_NR_ALLOCATIONS
     557 |         _compiletime_assert(condition, msg, __compiletime_assert_, __COUNTER__)
         |                                             ^
   include/linux/compiler_types.h:538:25: note: in definition of macro '__compiletime_assert'
     538 |                         prefix ## suffix();                             \
         |                         ^~~~~~
   include/linux/compiler_types.h:557:9: note: in expansion of macro '_compiletime_assert'
     557 |         _compiletime_assert(condition, msg, __compiletime_assert_, __COUNTER__)
         |         ^~~~~~~~~~~~~~~~~~~
   include/linux/build_bug.h:39:37: note: in expansion of macro 'compiletime_assert'
      39 | #define BUILD_BUG_ON_MSG(cond, msg) compiletime_assert(!(cond), msg)
         |                                     ^~~~~~~~~~~~~~~~~~
   include/linux/build_bug.h:50:9: note: in expansion of macro 'BUILD_BUG_ON_MSG'
      50 |         BUILD_BUG_ON_MSG(condition, "BUILD_BUG_ON failed: " #condition)
         |         ^~~~~~~~~~~~~~~~
   drivers/nvme/host/pci.c:3804:9: note: in expansion of macro 'BUILD_BUG_ON'
    3804 |         BUILD_BUG_ON(nvme_pci_npages_prp() > NVME_MAX_NR_ALLOCATIONS);
         |         ^~~~~~~~~~~~

Force it to be __always_inline to make sure it is always available for
use with BUILD_BUG_ON().

Reported-by: kernel test robot <lkp@intel.com>
Closes: https://lore.kernel.org/oe-kbuild-all/202505061846.12FMyRjj-lkp@intel.com/
Fixes: c372cdd1efdf ("nvme-pci: iod npages fits in s8")
Signed-off-by: Kees Cook <kees@kernel.org>
Signed-off-by: Christoph Hellwig <hch@lst.de>