nfs: Fix KMSAN warning in decode_getfattr_attrs()

nfs: Fix KMSAN warning in decode_getfattr_attrs()

Fix the following KMSAN warning:

CPU: 1 UID: 0 PID: 7651 Comm: cp Tainted: G    B
Tainted: [B]=BAD_PAGE
Hardware name: QEMU Standard PC (Q35 + ICH9, 2009)
=====================================================
=====================================================
BUG: KMSAN: uninit-value in decode_getfattr_attrs+0x2d6d/0x2f90
 decode_getfattr_attrs+0x2d6d/0x2f90
 decode_getfattr_generic+0x806/0xb00
 nfs4_xdr_dec_getattr+0x1de/0x240
 rpcauth_unwrap_resp_decode+0xab/0x100
 rpcauth_unwrap_resp+0x95/0xc0
 call_decode+0x4ff/0xb50
 __rpc_execute+0x57b/0x19d0
 rpc_execute+0x368/0x5e0
 rpc_run_task+0xcfe/0xee0
 nfs4_proc_getattr+0x5b5/0x990
 __nfs_revalidate_inode+0x477/0xd00
 nfs_access_get_cached+0x1021/0x1cc0
 nfs_do_access+0x9f/0xae0
 nfs_permission+0x1e4/0x8c0
 inode_permission+0x356/0x6c0
 link_path_walk+0x958/0x1330
 path_lookupat+0xce/0x6b0
 filename_lookup+0x23e/0x770
 vfs_statx+0xe7/0x970
 vfs_fstatat+0x1f2/0x2c0
 __se_sys_newfstatat+0x67/0x880
 __x64_sys_newfstatat+0xbd/0x120
 x64_sys_call+0x1826/0x3cf0
 do_syscall_64+0xd0/0x1b0
 entry_SYSCALL_64_after_hwframe+0x77/0x7f

The KMSAN warning is triggered in decode_getfattr_attrs(), when calling
decode_attr_mdsthreshold(). It appears that fattr->mdsthreshold is not
initialized.

Fix the issue by initializing fattr->mdsthreshold to NULL in
nfs_fattr_init().

Cc: [email protected] # v3.5.x
Fixes: 88034c3d88c2 ("NFSv4.1 mdsthreshold attribute xdr")
Signed-off-by: Roberto Sassu <[email protected]>
Signed-off-by: Anna Schumaker <[email protected]>