close_files(): don't bother with xchg()

At that point nobody else has references to the victim files_struct;
as the matter of fact, the caller will free it immediately after
close_files() returns, with no RCU delays or anything of that sort.

That's why we are not protecting against fdtable reallocation on
expansion, not cleaning the bitmaps, etc.  There's no point
zeroing the pointers in ->fd[] either, let alone make that an
atomic operation.

Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>