lib: cleanup kstrto*() usage

Use proper conversion functions.  kstrto*() variants exist for all
standard types.

Link: https://lkml.kernel.org/r/20201122123410.GB92364@localhost.localdomain
Signed-off-by: Alexey Dobriyan <adobriyan@gmail.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>