lib/cpumask: move trivial wrappers around find_bit to the header

To avoid circular dependencies, cpumask keeps simple (almost) one-line
wrappers around find_bit() in a c-file.

Commit 47d8c156 ("include: move find.h from asm_generic to linux")
moved find.h header out of asm_generic include path, and it helped to fix
many circular dependencies, including some in cpumask.h.

This patch moves those one-liners to header files.

Signed-off-by: Yury Norov <yury.norov@gmail.com>