[XFS] Fix xfs_splice_write() so appended data gets to disk.

xfs_splice_write() failed to update the on disk inode size when extending
the so when the file was closed the range extended by splice was truncated
off. Hence any region of a file written to by splice would end up as a
hole full of zeros.

SGI-PV: 955939
SGI-Modid: xfs-linux-melb:xfs-kern:26920a

Signed-off-by: David Chinner <dgc@sgi.com>
Signed-off-by: David Chatterton <chatz@sgi.com>