new helper: daemonize_descriptors()

descriptor-related parts of daemonize, done right.  As the
result we simplify the locking rules for ->files - we
hold task_lock in *all* cases when we modify ->files.

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

diff --cc fs/file.c
Simple merge

diff --cc include/linux/fdtable.h
Simple merge

diff --cc kernel/exit.c
Simple merge