xfs: rename the whichfork variable in xfs_buffered_write_iomap_begin

Renaming whichfork to allocfork in xfs_buffered_write_iomap_begin makes
the usage of this variable a little more clear.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Allison Collins <allison.henderson@oracle.com>
Reviewed-by: Darrick J. Wong <darrick.wong@oracle.com>
Signed-off-by: Darrick J. Wong <darrick.wong@oracle.com>