autofs4: don't open-code fd_install()

The only difference between autofs_dev_ioctl_fd_install() and
fd_install() is __set_close_on_exec() done by the latter.  Just
use get_unused_fd_flags(O_CLOEXEC) to allocate the descriptor
and be done with that...

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