xen/blkfront: Only check REQ_FUA for writes

The existing code silently converts read operations with the
REQ_FUA bit set into write-barrier operations. This results in data
loss as the backend scribbles zeroes over the data instead of returning
it.

While the REQ_FUA bit doesn't make sense on a read operation, at least
one well-known out-of-tree kernel module does set it and since it
results in data loss, let's be safe here and only look at REQ_FUA for
writes.

Signed-off-by: Ross Lagerwall <ross.lagerwall@citrix.com>
Acked-by: Juergen Gross <jgross@suse.com>
Link: https://lore.kernel.org/r/20230426164005.2213139-1-ross.lagerwall@citrix.com
Signed-off-by: Juergen Gross <jgross@suse.com>

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