ext4: Add iomap support for inline data

Report inline data as a IOMAP_F_DATA_INLINE mapping.  This allows to use
iomap_seek_hole and iomap_seek_data in ext4_llseek and makes switching
to iomap_fiemap in ext4_fiemap easier.

Signed-off-by: Andreas Gruenbacher <agruenba@redhat.com>
Signed-off-by: Theodore Ts'o <tytso@mit.edu>
Reviewed-by: Jan Kara <jack@suse.cz>