ext4: Add a uapi header for ext4 userspace APIs

Create a uapi header include/uapi/linux/ext4.h, move the ioctls and
associated data structures to the uapi header, and include it from
fs/ext4/ext4.h.

Signed-off-by: Josh Triplett <josh@joshtriplett.org>
Link: https://lore.kernel.org/r/680175260970d977d16b5cc7e7606483ec99eb63.1680402881.git.josh@joshtriplett.org
Signed-off-by: Theodore Ts'o <tytso@mit.edu>