[XFS] stop re-checking permissions in xfs_swapext

xfs_swapext should simplify check if we have a writeable file descriptor
instead of re-checking the permissions using xfs_iaccess. Add an
additional check to refuse O_APPEND file descriptors because swapext is
not an append-only write operation.

SGI-PV: 971186
SGI-Modid: xfs-linux-melb:xfs-kern:30369a

Signed-off-by: Christoph Hellwig <hch@infradead.org>
Signed-off-by: Lachlan McIlroy <lachlan@sgi.com>