ext2fs: move calculate_summary_stats to ext2fs lib

ext2fs: move calculate_summary_stats to ext2fs lib

The function calculate_summary_stats sets the global metadata of the
file system. Tune2fs had this function defined statically in
tune2fs.c. Fast commit replay needs this function to set global
metadata at the end of the replay phase. So, move this function to
libext2fs.

Signed-off-by: Harshad Shirwadkar <harshadshirwadkar@gmail.com>
Reviewed-by: Theodore Ts'o <tytso@mit.edu>
Signed-off-by: Theodore Ts'o <tytso@mit.edu>