mm/huge_memory.c: fix "orig_pud" set but not used

mm/huge_memory.c: fix "orig_pud" set but not used

Commit a00cc7d9dd93 ("mm, x86: add support for PUD-sized transparent
hugepages") introduced pudp_huge_get_and_clear_full() but no one uses
its return code.

In order to not diverge from pmdp_huge_get_and_clear_full(), just change
zap_huge_pud() to not assign the return value from
pudp_huge_get_and_clear_full().

  mm/huge_memory.c: In function 'zap_huge_pud':
  mm/huge_memory.c:1982:8: warning: variable 'orig_pud' set but not used [-Wunused-but-set-variable]
    pud_t orig_pud;
          ^~~~~~~~

Link: http://lkml.kernel.org/r/20190301221956.97493-1-cai@lca.pw
Signed-off-by: Qian Cai <cai@lca.pw>
Reviewed-by: Andrew Morton <akpm@linux-foundation.org>
Cc: Matthew Wilcox <willy@infradead.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>