xfs: simplify local variable assignment in file write code

Get the struct inode pointer from iocb->ki_filp->f_mapping->host
directly and the other variables are unnecessary, so simplify the
local variables assignment.

Signed-off-by: Kaixu Xia <kaixuxia@tencent.com>
Reviewed-by: Darrick J. Wong <djwong@kernel.org>
Signed-off-by: Dave Chinner <david@fromorbit.com>