xfs: xfs_iomap_prealloc_size() tracepoint

Add a tracepoint to provide some feedback on preallocation size
calculation.

Signed-off-by: Brian Foster <bfoster@redhat.com>
Reviewed-by: Mark Tinguely <tinguely@sgi.com>
Signed-off-by: Ben Myers <bpm@sgi.com>

diff --git a/fs/xfs/xfs_iomap.c b/fs/xfs/xfs_iomap.c
index 0a53085..8f8aaee 100644
--- a/fs/xfs/xfs_iomap.c
+++ b/fs/xfs/xfs_iomap.c
@@ -518,6 +518,9 @@
 	if (alloc_blocks < mp->m_writeio_blocks)
 		alloc_blocks = mp->m_writeio_blocks;
 
+	trace_xfs_iomap_prealloc_size(ip, alloc_blocks, shift,
+				      mp->m_writeio_blocks);
+
 	return alloc_blocks;
 }