iomap: improve the warnings from iomap_swapfile_activate

Print the path name of the swapfile that failed to active to ease
debugging the problem and to avoid a scare if xfstests hits these
cases.  Also reword one warning a bit, as the error is not about
a file being on multiple devices, but one that has at least an
extent outside the main device known to the VFS and swap code.

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