iomap: provide more useful errors for invalid swap files

iomap: provide more useful errors for invalid swap files

Currently, for an invalid swap file, we print the same error message
regardless of the reason. This isn't very useful for an admin, who will
likely want to know why exactly they can't use their swap file. So,
let's add specific error messages for each reason, and also move the
bdev check after the flags checks, since the latter are more
fundamental.

Reviewed-by: Darrick J. Wong <[email protected]>
Signed-off-by: Omar Sandoval <[email protected]>
Reviewed-by: Christoph Hellwig <[email protected]>
Reviewed-by: Jan Kara <[email protected]>
Signed-off-by: Darrick J. Wong <[email protected]>