xfs: prevent swapext from operating on write-only files

commit 1817176a86352f65210139d4c794ad2d19fc6b63 upstream.

This patch prevents user "foo" from using the SWAPEXT ioctl to swap
a write-only file owned by user "bar" into a file owned by "foo" and
subsequently reading it.  It does so by checking that the file
descriptors passed to the ioctl are also opened for reading.

Signed-off-by: Dan Rosenberg <dan.j.rosenberg@gmail.com>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>