tune2fs: Improve the error messages for tune2fs -I

tune2fs: Improve the error messages for tune2fs -I

If the current inode size is less than or equal to the requested inode
size, either explain that shrinking the inode size is not supported,
or that the inode is already the requested size.  Also, open the
filesystem provisionally first to do the inode size check and don't
setup up the undo log until we know that we're actually going to
perform the inode resizing operation.

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