PCI: Fix BUILD_BUG_ON usage for old gcc

PCI: Fix BUILD_BUG_ON usage for old gcc

As reported in the below link, it seems older versions of gcc cannot
determine that the howmany variable is known for all callers.  Include
a test so that newer compilers can enforce this sanity check and older
compilers can still work.  Add __always_inline attribute to give the
compiler an even better chance to know the inputs.

Link: https://lore.kernel.org/r/20250212185337.293023-1-alex.williamson@redhat.com
Fixes: 4453f360862e ("PCI: Batch BAR sizing operations")
Reported-by: Oleg Nesterov <oleg@redhat.com>
Link: https://lore.kernel.org/all/20250209154512.GA18688@redhat.com
Signed-off-by: Alex Williamson <alex.williamson@redhat.com>
Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>
Tested-by: Oleg Nesterov <oleg@redhat.com>
Tested-by: Mitchell Augustin <mitchell.augustin@canonical.com>