tune2fs: speed up rewriting extent tree when enabling metadata_csum

tune2fs: speed up rewriting extent tree when enabling metadata_csum

When enabling checksums, tune2fs naively rewrites every extent in the
entire tree!  This is unnecessary since we only need to rewrite each
extent tree block; therefore, only rewrite the extent if it's the
first one in an internal extent tree block.

Also, don't bother iterating the extent tree when clearing checksums.

Signed-off-by: Darrick J. Wong <darrick.wong@oracle.com>
Signed-off-by: Theodore Ts'o <tytso@mit.edu>