include/linux/huge_mm.h: return NULL instead of false for pmd_trans_huge_lock()

The return value of pmd_trans_huge_lock() is a pointer, not a boolean
value, so use NULL instead of false as the return value.

Signed-off-by: Chen Gang <gang.chen.5i5j@gmail.com>
Acked-by: Kirill A. Shutemov <kirill.shutemov@linux.intel.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>