tune2fs: fix false warning that a UUID change will take a long time

tune2fs: fix false warning that a UUID change will take a long time

If the file system only has the flex_bg feature enabled (with out the
metadata_csum feature enabled), it won't take a long time time fix up
the checksums after changing the UUID.  While it does need to
recalculate all of the checksums in the block group descriptors, that
doesn't take a long time.

Also, if the ea_data feature is enabled, changing the UUID will also
take a long time, and we weren't warning the user about that case.

Fix up the warning message so it doesn't mislead people, and is more
accurate.

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