[LINUX] Fix clear_fixmap().

[LINUX] Fix clear_fixmap().

On i386-PAE, clear_fixmap() results in ill use of set_pte(). In all
contexts, p2m translations shouldn't occur here. Note that this is not
really an issue on native linux, as there is
(a) no pfn-to-mfn translation and
(b) __set_fixmap() takes an unsigned long physical address rather than
    a paddr_t, which makes it so that bits 32 and up of the physical
    address are always zero, permitting either order store when
    clearing the entry.

Signed-off-by: Jan Beulich <jbeulich@novell.com>