f2fs: make LAZYTIME a mount option flag

Set LAZYTIME into sbi during parsing, and transfer it to the sb in
fill_super, so that an sb is not required during option parsing.

(Note: While lazytime is normally handled via mount flag in the vfs,
some f2fs users do expect to be able to use it as an explicit mount
option string via the mount syscall, so this option must remain.)

Signed-off-by: Eric Sandeen <sandeen@redhat.com>
Reviewed-by: Chao Yu <chao@kernel.org>
Signed-off-by: Jaegeuk Kim <jaegeuk@kernel.org>