MAINTAINERS: Clarify UIO vs UIOVEC maintainer

The UIO file mask in MAINTAINERS was incorrectly directing UIOVEC
(include/linux/uio.h) patches to Greg.

Tag Al as the UIOVEC maintainer as Ingo and others have explicitly
required his ack before taking architecture patches that touch
lib/iov_iter.c.

Cc: Al Viro <viro@zeniv.linux.org.uk>
Reported-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Signed-off-by: Dan Williams <dan.j.williams@intel.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>