Change resize2fs open modes for mounted and unmounted filesystems

Change resize2fs open modes for mounted and unmounted filesystems

If the filesystem is mounted, open it in read-only mode since the userspace
program should not try to modify it directly.  If the filesystem is not mounted,
open it in exclusive mode to avoid potential problems (such as someone
trying to mount the filesystem while it is being resized).

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