linkage.h: fix build breakage due to symbol prefix handling

Al's commit e1b5bb6d ("consolidate cond_syscall and SYSCALL_ALIAS
declarations") broke the build on blackfin and metag due to the
following code:

  #ifndef SYMBOL_NAME
  #ifdef CONFIG_SYMBOL_PREFIX
  #define SYMBOL_NAME(x) CONFIG_SYMBOL_PREFIX ## x
  #else
  #define SYMBOL_NAME(x) x
  #endif
  #endif
  #define __SYMBOL_NAME(x) __stringify(SYMBOL_NAME(x))

__stringify literally stringifies CONFIG_SYMBOL_PREFIX ##x, so you get
lines like this in kernel/sys_ni.s:

  .weak CONFIG_SYMBOL_PREFIXsys_quotactl
  .set CONFIG_SYMBOL_PREFIXsys_quotactl,CONFIG_SYMBOL_PREFIXsys_ni_syscall

The patches in Rusty's modules-next tree such as "CONFIG_SYMBOL_PREFIX:
cleanup." cleans up the whole mess around symbol prefixes, so this patch
just attempts to fix the build in the meantime.

The intermediate definition of SYMBOL_NAME above isn't used and is
incorrect when CONFIG_SYMBOL_PREFIX is defined as CONFIG_SYMBOL_PREFIX
is a quoted string literal, so define __SYMBOL_NAME directly depending
on CONFIG_SYMBOL_PREFIX.

Signed-off-by: James Hogan <james.hogan@imgtec.com>
Mea-culpa-by: Al Viro <viro@zeniv.linux.org.uk>
Cc: Rusty Russell <rusty@rustcorp.com.au>
Cc: Mike Frysinger <vapier@gentoo.org>
Cc: uclinux-dist-devel@blackfin.uclinux.org
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>