ext4: fix flags validity checking for EXT4_IOC_CHECKPOINT

Use the correct bitmask when checking for any not-yet-supported flags.

Link: https://lore.kernel.org/r/20210702173425.1276158-1-tytso@mit.edu


Fixes: 351a0a3f ("ext4: add ioctl EXT4_IOC_CHECKPOINT")
Signed-off-by: Theodore Ts'o <tytso@mit.edu>
Reviewed-by: Leah Rumancik <leah.rumancik@gmail.com>