tune2fs: allow changing the UUID mounted file systems with the -f option

tune2fs: allow changing the UUID mounted file systems with the -f option

There are times when it is necessary to update the UUID on a mounted
root file system (for example).  So when we add this this safety check
to e2fsprogs 1.43, we will likely break some scripts.  Allow the -f
option to force an override of this safety check.

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