string: on strstrip(), first remove leading spaces before running over str

... so that strlen() iterates over a smaller string comprising of the
remaining characters only.

Signed-off-by: André Goddard Rosa <andre.goddard@gmail.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>