Fix mistake in XLOG_SEG_SIZE test.

Fix mistake in XLOG_SEG_SIZE test.

The intent of the test is to check whether XLOG_SEG_SIZE is in a
particular range, but actually in one case it compares XLOG_BLCKSZ
by mistake.  Repair.

Commit 88e982302684246e8af785e78a467ac37c76dee9 introduced this
faulty test.

Kuntal Ghosh, reviewed by Michael Paquier.