drivers/md/md.c: use strreplace()

There's no point in starting over when we meet a '/'.  This also
eliminates a stack variable and a little .text.

Signed-off-by: Rasmus Villemoes <linux@rasmusvillemoes.dk>
Acked-by: NeilBrown <neilb@suse.de>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>