ext4: move inode extension check out from ext4_iomap_alloc()

Lift the inode extension/orphan list handling code out from
ext4_iomap_alloc() and apply it within the ext4_dax_write_iter().

Signed-off-by: Matthew Bobrowski <mbobrowski@mbobrowski.org>
Reviewed-by: Jan Kara <jack@suse.cz>
Reviewed-by: Ritesh Harjani <riteshh@linux.ibm.com>
Link: https://lore.kernel.org/r/fd5c84db25d5d0da87d97ed4c36fd844f57da759.1572949325.git.mbobrowski@mbobrowski.org


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