When CONFIG_HIGHMEM=y, but ZONE_NORMAL isn't quite full, there is, of
course, no actual memory at *high_memory.  This isn't a problem with normal
virt<->phys translations because it's never dereferenced, but
CONFIG_NONLINEAR is a bit more finicky.  So, don't do virt_to_phys() to
non-existent addresses.
Signed-off-by: Dave Hansen <haveblue@us.ibm.com>
Signed-off-by: Andrew Morton <akpm@osdl.org>
Signed-off-by: Linus Torvalds <torvalds@osdl.org>