iomap: refactor iomap_iter() length check and tracepoint

iomap_iter() checks iomap.length to skip individual code blocks not
appropriate for the initial case where there is no mapping in the
iter. To prepare for upcoming changes, refactor the code to jump
straight to the ->iomap_begin() handler in the initial case and move
the tracepoint to the top of the function so it always executes.

Signed-off-by: Brian Foster <bfoster@redhat.com>
Link: https://lore.kernel.org/r/20250207143253.314068-4-bfoster@redhat.com


Reviewed-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: "Darrick J. Wong" <djwong@kernel.org>
Signed-off-by: Christian Brauner <brauner@kernel.org>