ext4: add IOMAP_F_MERGED for non-extent based mapping

IOMAP_F_MERGED needs to be set in case of non-extent based mapping.
This is needed in later patches for conversion of ext4_fiemap to use iomap.

Reviewed-by: Darrick J. Wong <darrick.wong@oracle.com>
Reviewed-by: Jan Kara <jack@suse.cz>
Signed-off-by: Ritesh Harjani <riteshh@linux.ibm.com>
Link: https://lore.kernel.org/r/a4764c91c08c16d4d4a4b36defb2a08625b0e9b3.1582880246.git.riteshh@linux.ibm.com
Signed-off-by: Theodore Ts'o <tytso@mit.edu>