fs/Kconfig: move nilfs2 out

fs/Kconfig file was split into individual fs/*/Kconfig files before
nilfs was merged.  I've found the current config entry of nilfs is
tainting the work.  Sorry, I didn't notice.  This fixes the violation.

Signed-off-by: Ryusuke Konishi <konishi.ryusuke@lab.ntt.co.jp>
Cc: Alexey Dobriyan <adobriyan@gmail.com>