ext4: fix a copy and paste typo

This was obviously supposed to be an ext4 struct, not xfs.  GCC
doesn't care either way so it doesn't affect the build or runtime.

Fixes: cebe85d5 ("ext4: switch to the new mount api")
Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Reviewed-by: Lukas Czerner <lczerner@redhat.com>
Link: https://lore.kernel.org/r/20211215114309.GB14552@kili


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