ext4: Remove duplicate call to ext4_commit_super() in ext4_freeze()

Commit c4be0c1d added error checking to ext4_freeze() when calling
ext4_commit_super().  Unfortunately the patch failed to remove the
original call to ext4_commit_super(), with the net result that when
freezing the filesystem, the superblock gets written twice, the first
time without error checking.

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