WB-related sysfs entries can be accessed even when an UFS device does not
support the feature. The descriptors which are not supported by the UFS
device may be wrongly reported when they are accessed from their
corrsponding sysfs entries. Fix it by adding a sanity check of parameter
offset against the actual decriptor length.

Link: https://lore.kernel.org/r/1603346348-14149-1-git-send-email-cang@codeaurora.orgReviewed-by: NAsutosh Das <asutoshd@codeaurora.org>
Acked-by: NDaejun Park <daejun7.park@samsung.com>
Signed-off-by: NCan Guo <cang@codeaurora.org>
Signed-off-by: NMartin K. Petersen <martin.petersen@oracle.com>