xfs: xfs_trans_read_buf() should return an error on failure

When inside a transaction and we fail to read a buffer,
xfs_trans_read_buf returns a null buffer pointer and no error.
xfs_do_da_buf() checks the error return, but not the buffer, and as
a result this read failure condition causes a panic when it attempts
to dereference the non-existant buffer.

Make xfs_trans_read_buf() return the same error for this situation
regardless of whether it is in a transaction or not. This means
every caller does not need to check both the error return and the
buffer before proceeding to use the buffer.
Signed-off-by: Dave Chinner <dchinner@redhat.com>
Reviewed-by: Alex Elder <aelder@sgi.com>

fs/xfs/xfs_trans_buf.c

@@ -383,7 +383,8 @@ xfs_trans_read_buf(
 	bp = xfs_buf_read(target, blkno, len, flags | XBF_DONT_BLOCK);
 	if (bp == NULL) {
 		*bpp = NULL;
-		return 0;
+		return (flags & XBF_TRYLOCK) ?
+					0 : XFS_ERROR(ENOMEM);
 	}
 	if (XFS_BUF_GETERROR(bp) != 0) {
 	    XFS_BUF_SUPER_STALE(bp);