shift current_cred() from __f_setown() to f_modown()

Shift current_cred() from __f_setown() to f_modown(). This reduces
the number of arguments and saves 48 bytes from fs/fcntl.o.

[ Note: this doesn't clear euid/uid when pid is set to NULL.  But if
  f_owner.pid == NULL we never use f_owner.uid/euid.  Otherwise we'd
  have a bug anyway: we must not send signals if pid was reset to NULL.  ]

Signed-off-by: Oleg Nesterov <oleg@redhat.com>
Acked-by: David Howells <dhowells@redhat.com>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>