NFSv4.1: work around -Wmaybe-uninitialized warning

A bugfix introduced a harmless gcc warning in nfs4_slot_seqid_in_use
if we enable -Wmaybe-uninitialized again:

fs/nfs/nfs4session.c:203:54: error: 'cur_seq' may be used uninitialized in this function [-Werror=maybe-uninitialized]

gcc is not smart enough to conclude that the IS_ERR/PTR_ERR pair
results in a nonzero return value here. Using PTR_ERR_OR_ZERO()
instead makes this clear to the compiler.

The warning originally did not appear in v4.8 as it was globally
disabled, but the bugfix that introduced the warning got backported
to stable kernels which again enable it, and this is now the only
warning in the v4.7 builds.

Fixes: e09c978a ("NFSv4.1: Fix Oopsable condition in server callback races")
Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Cc: Trond Myklebust <trond.myklebust@primarydata.com>
Signed-off-by: Anna Schumaker <Anna.Schumaker@Netapp.com>