iomap: iomap_read_inline_data cleanup

Change iomap_read_inline_data to return 0 or an error code; this
simplifies the callers.  Add a description.

Signed-off-by: Andreas Gruenbacher <agruenba@redhat.com>
Reviewed-by: Christoph Hellwig <hch@lst.de>
[djwong: document the return value of iomap_read_inline_data explicitly]
Reviewed-by: Darrick J. Wong <djwong@kernel.org>
Signed-off-by: Darrick J. Wong <djwong@kernel.org>