ext4: split _ext4_fiemap

The fiemap and EXT4_IOC_GET_ES_CACHE cases share almost no code, so split
them into entirely separate functions.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Ritesh Harjani <riteshh@linux.ibm.com>
Reviewed-by: Jan Kara <jack@suse.cz>
Link: https://lore.kernel.org/r/20200523073016.2944131-2-hch@lst.de


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