um: Replace to_phys() and to_virt() with less generic function names

The UML function names to_virt() and to_phys() are exposed by UML
headers, and are very generic and may be defined by drivers.  As it
turns out, commit 9409c9b6 ("pmem: refactor pmem_clear_poison()")
did exactly that.

This results in build errors such as the following when trying to build
um:allmodconfig:

  drivers/nvdimm/pmem.c: In function ‘pmem_dax_zero_page_range’:
  ./arch/um/include/asm/page.h:105:20: error: too few arguments to function ‘to_phys’
    105 | #define __pa(virt) to_phys((void *) (unsigned long) (virt))
        |                    ^~~~~~~

Use less generic function names for the um specific to_phys() and
to_virt() functions to fix the problem and to avoid similar problems in
the future.

Fixes: 9409c9b6 ("pmem: refactor pmem_clear_poison()")
Cc: Dan Williams <dan.j.williams@intel.com>
Cc: Christoph Hellwig <hch@lst.de>
Signed-off-by: Guenter Roeck <linux@roeck-us.net>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>