md: factor out parsing of fixed-point numbers

safe_delay_store can parse fixed point numbers (for fractions
of a second).  We will want to do that for another sysfs
file soon, so factor out the code.

Signed-off-by: NeilBrown <neilb@suse.de>

diff --cc drivers/md/md.c
Simple merge

diff --cc drivers/md/md.h
Simple merge