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 <[email protected]>
Signed-off-by: Wang Jianchao <[email protected]>
Signed-off-by: Chandan Babu R <[email protected]>

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