ubi: Use PAGE_SIZE as size field for ubi sysfs show attributes

To avoid truncation and to show correct ubi sysfs show attribute
values, use PAGE_SIZE as a buffer size value.

Change-Id: I4cf84967437b7baae0f10f00fd3d8726f780af16
Signed-off-by: Pradeep P V K <quic_pragalla@quicinc.com>