f2fs: clean up new_curseg()

Move f2fs_valid_pinned_area() check logic from new_curseg() to
get_new_segment(), it can avoid calling __set_free() if it fails
to find free segment in conventional zone for pinned file.

Signed-off-by: Chao Yu <chao@kernel.org>
Reviewed-by: Daeho Jeong <daehojeong@google.com>
Signed-off-by: Jaegeuk Kim <jaegeuk@kernel.org>