bs_sheepdog.c: fix gcc warning at sd_open()

This fixes the following warning at sd_open function, which happens
because gcc isn't always perfect with determining whether a variable
is used uninitialized:

cc -c -DUSE_SIGNALFD -DUSE_TIMERFD -D_GNU_SOURCE -I. -g -O2
-fno-strict-aliasing -Wall -Wstrict-prototypes -fPIC DTGT_VERSION=\"\"
--DBSDIR=\"/lib/tgt/backing-store\" bs_sheepdog.c o bs_sheepdog.o
bs_sheepdog.c: In function 'sd_open':
bs_sheepdog.c:920: warning: 'saveptr' may be used uninitialized in
this function

Signed-off-by: Ryusuke Konishi <konishi.ryusuke@lab.ntt.co.jp>
Signed-off-by: FUJITA Tomonori <fujita.tomonori@lab.ntt.co.jp>
bs_sheepdog.c: fix length modifier of eprintf in read_write_object()

On 32-bit environments, the sheepdog driver causes the following
compile warning because "%lx" is used for uint64_t type variables:

cc -c -DUSE_SIGNALFD -DUSE_TIMERFD -D_GNU_SOURCE -I. -g -O2
-fno-strict-aliasing -Wall -Wstrict-prototypes -fPIC
-DTGT_VERSION=\"\" -DBSDIR=\"/lib/tgt/backing-store\" bs_sheepdog.c -o
bs_sheepdog.o
bs_sheepdog.c: In function 'read_write_object':
bs_sheepdog.c:682: warning: format '%lx' expects type 'long unsigned
int', but argument 5 has type 'uint64_t'
bs_sheepdog.c:682: warning: format '%lx' expects type 'long unsigned
int', but argument 6 has type 'uint64_t'

This fixes the issue by using PRIx64 length modifier instead of "%lx".

Signed-off-by: Ryusuke Konishi <konishi.ryusuke@lab.ntt.co.jp>
Cc: Hitoshi Mitake <mitake.hitoshi@lab.ntt.co.jp>
Signed-off-by: FUJITA Tomonori <fujita.tomonori@lab.ntt.co.jp>