support/testing: test_jffs2.py: update logical eraseblock size for qemu >= 2.9

The test_jffs2 test fail for the same reason as test_ubi test with qemu >= 2.9
due to a qemu 2.8 bug. See commit d8447c38f53f98e1df4a209e9f70e54f8b60f36e.

Divide the erase block size by two.

Fixes:
https://gitlab.com/buildroot.org/buildroot/-/jobs/1687590514

Signed-off-by: Romain Naour <romain.naour@gmail.com>
Signed-off-by: Yann E. MORIN <yann.morin.1998@free.fr>

support/testing/tests/fs/test_jffs2.py

@@ -16,7 +16,7 @@ class TestJffs2(infra.basetest.BRTest):
         """
         BR2_TARGET_ROOTFS_JFFS2=y
         BR2_TARGET_ROOTFS_JFFS2_CUSTOM=y
-        BR2_TARGET_ROOTFS_JFFS2_CUSTOM_EBSIZE=0x80000
+        BR2_TARGET_ROOTFS_JFFS2_CUSTOM_EBSIZE=0x40000
         BR2_TARGET_ROOTFS_JFFS2_NOCLEANMARKER=y
         BR2_TARGET_ROOTFS_JFFS2_PAD=y
         BR2_TARGET_ROOTFS_JFFS2_PADSIZE=0x4000000