vfs: clear remainder of 'full_fds_bits' in dup_fd()

This fixes a bug from commit f3f86e33 ("vfs: Fix pathological
performance case for __alloc_fd()").

v2: refactor to share fd bitmap copying code
Signed-off-by: Eric Biggers <ebiggers3@gmail.com>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>