target/file: Fix UNMAP with DIF protection support

When UNMAP command is issued with DIF protection support enabled,
the protection info for the unmapped region is remain unchanged.
So READ command for the region causes data integrity failure.

This fixes it by invalidating protection info for the unmapped region
by filling with 0xff pattern.  This change also adds helper function
fd_do_prot_fill() in order to reduce code duplication with existing
fd_format_prot().

Signed-off-by: Akinobu Mita <akinobu.mita@gmail.com>
Reviewed-by: Sagi Grimberg <sagig@mellanox.com>
Reviewed-by: "Martin K. Petersen" <martin.petersen@oracle.com>
Cc: Christoph Hellwig <hch@lst.de>
Cc: "James E.J. Bottomley" <James.Bottomley@HansenPartnership.com>
Cc: <stable@vger.kernel.org> # v3.14+
Signed-off-by: Nicholas Bellinger <nab@linux-iscsi.org>