ext4: move ext4_fiemap to use iomap framework

This patch moves ext4_fiemap to use iomap framework.
For xattr a new 'ext4_iomap_xattr_ops' is added.

Reported-by: kbuild test robot <lkp@intel.com>
Reviewed-by: Jan Kara <jack@suse.cz>
Reviewed-by: Darrick J. Wong <darrick.wong@oracle.com>
Link: https://lore.kernel.org/r/b9f45c885814fcdd0631747ff0fe08886270828c.1582880246.git.riteshh@linux.ibm.com


Signed-off-by: Ritesh Harjani <riteshh@linux.ibm.com>
Signed-off-by: Theodore Ts'o <tytso@mit.edu>