uml: use simple_write_to_buffer()

Simplify write file operation for mmapper by using
simple_write_to_buffer().

Signed-off-by: Akinobu Mita <akinobu.mita@gmail.com>
Cc: Jeff Dike <jdike@addtoit.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>