xfs: use roundup_pow_of_two instead of ffs during xlog_find_tail

In our production environment, we find that mounting a 500M /boot
which is umount cleanly needs ~6s. One cause is that ffs() is
used by xlog_write_log_records() to decide the buffer size. It
can cause a lot of small IO easily when xlog_clear_stale_blocks()
needs to wrap around the end of log area and log head block is
not power of two. Things are similar in xlog_find_verify_cycle().

The code is able to handed bigger buffer very well, we can use
roundup_pow_of_two() to replace ffs() directly to avoid small
and sychronous IOs.
Reviewed-by: Dave Chinner <dchinner@redhat.com>
Signed-off-by: Wang Jianchao <wangjc136@midea.com>
Signed-off-by: Chandan Babu R <chandanbabu@kernel.org>

fs/xfs/xfs_log_recover.c

@@ -329,7 +329,7 @@ xlog_find_verify_cycle(
 	 * try a smaller size.  We need to be able to read at least
 	 * a log sector, or we're out of luck.
 	 */
-	bufblks = 1 << ffs(nbblks);
+	bufblks = roundup_pow_of_two(nbblks);
 	while (bufblks > log->l_logBBsize)
 		bufblks >>= 1;
 	while (!(buffer = xlog_alloc_buffer(log, bufblks))) {
@@ -1528,7 +1528,7 @@ xlog_write_log_records(
 	 * a smaller size.  We need to be able to write at least a
 	 * log sector, or we're out of luck.
 	 */
-	bufblks = 1 << ffs(blocks);
+	bufblks = roundup_pow_of_two(blocks);
 	while (bufblks > log->l_logBBsize)
 		bufblks >>= 1;
 	while (!(buffer = xlog_alloc_buffer(log, bufblks))) {