libext2fs: refactor code which fixes up the checksums in an extent tree

libext2fs: refactor code which fixes up the checksums in an extent tree

The code to recalculate the checksums in an extent tree (which is
needed after an inode is relocated so it has a different inode number)
was duplicated in tune2fs and resize2fs.  In addition, this work could
be done in a much more efficient way inside lib/ext2fs/extent.c.

This commit creates a new library function which corrects the
checksums in an inode's extent tree, named: ext2fs_fix_extents_checksums()

Signed-off-by: Theodore Ts'o <tytso@mit.edu>