iomap: constrain the file range passed to iomap_file_unshare

iomap: constrain the file range passed to iomap_file_unshare

File contents can only be shared (i.e. reflinked) below EOF, so it makes
no sense to try to unshare ranges beyond EOF.  Constrain the file range
parameters here so that we don't have to do that in the callers.

Fixes: 5f4e5752a8a3 ("fs: add iomap_file_dirty")
Signed-off-by: Darrick J. Wong <djwong@kernel.org>
Link: https://lore.kernel.org/r/20241002150213.GC21853@frogsfrogsfrogs
Reviewed-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Brian Foster <bfoster@redhat.com>
Signed-off-by: Christian Brauner <brauner@kernel.org>