librbd/cache/pwl: WriteLogCacheEntry constructor must initialize flags

librbd/cache/pwl: WriteLogCacheEntry constructor must initialize flags

Initializing the individual bit field members leaves the remaining two
bits uninitialized and that garbage state gets persisted.

In general, using bit fields in a structure where the layout actually
matters is not desirable.  Even with a few single bits, such as here,
their order, strictly speaking, is not guaranteed:

    An implementation may allocate any addressable storage unit large
    enough to hold a bit-field. If enough space remains, a bit-field
    that immediately follows another bit-field in a structure shall be
    packed into adjacent bits of the same unit. If insufficient space
    remains, whether a bit-field that does not fit is put into the next
    unit or overlaps adjacent units is implementation-defined. The
    order of allocation of bit-fields within a unit (high-order to
    low-order or low-order to high-order) is implementation-defined.
    The alignment of the addressable storage unit is unspecified.

Signed-off-by: Ilya Dryomov <idryomov@gmail.com>
(cherry picked from commit 91d270b210a908ea2f3578dd7db3263383da95a8)