libxfs: turn the xfs_buf_incore stub into an inline function

Replace the macro with an inline function to avoid compiler warnings with new
backports of kernel code.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Darrick J. Wong <darrick.wong@oracle.com>
Signed-off-by: Eric Sandeen <sandeen@sandeen.net>

diff --git a/libxfs/libxfs_priv.h b/libxfs/libxfs_priv.h
index 6d5e476..3cd2be5 100644
--- a/libxfs/libxfs_priv.h
+++ b/libxfs/libxfs_priv.h
@@ -371,8 +371,12 @@
 #define XFS_BUF_UNDELAYWRITE(bp)	((bp)->b_flags &= ~LIBXFS_B_DIRTY)
 #define XFS_BUF_SET_BDSTRAT_FUNC(a,b)	((void) 0)
 
-/* avoid gcc warning */
-#define xfs_buf_incore(bt,blkno,len,lockit)	NULL
+static inline struct xfs_buf *xfs_buf_incore(struct xfs_buftarg *target,
+		xfs_daddr_t blkno, size_t numblks, xfs_buf_flags_t flags)
+{
+	return NULL;
+}
+
 #define xfs_buf_oneshot(bp)		((void) 0)
 
 #define XBRW_READ			LIBXFS_BREAD