alpha: regularize the situation with asm/param.h

alpha: regularize the situation with asm/param.h

The only reason why alpha can't do what sparc et.al. are doing
is that include/asm-generic/param.h relies upon the value of HZ
set for userland header in uapi/asm/param.h being 100.

We need that value to define USER_HZ and we need that definition
to outlive the redefinition of HZ kernel-side.  And alpha needs
it to be 1024, not 100 like everybody else.

So let's add __USER_HZ to uapi/asm-generic/param.h, defaulting to
100 and used to define HZ.  That way include/asm-generic/param.h
can use that thing instead of open-coding it - it won't be affected
by undefining and redefining HZ.

That done, alpha asm/param.h can be removed and uapi/asm/param.h
switched to defining __USER_HZ and EXEC_PAGESIZE and then including
<asm-generic/param.h> - asm/param.h will resolve to uapi/asm/param.h,
which pulls <asm-generic/param.h>, which will do the right thing
both in the kernel and userland contexts.

Reviewed-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>