f2fs: add static to get_max_meta_blks

inline get_max_meta_blks is only used in checkpoint.c
Use standard static inline format.

Cc: Jaegeuk Kim <jaegeuk.kim@samsung.com>
Cc: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Fabian Frederick <fabf@skynet.be>
Signed-off-by: Jaegeuk Kim <jaegeuk.kim@samsung.com>