nfsroot: make nfsroot to accept the 1024 bytes long directory name

nfsroot: make nfsroot to accept the 1024 bytes long directory name

although NFS_MAXPATHLEN is defined to 1024, nfs client hopes to accept
a 1024 byte path, but nfs_root_parms is limited to 256, and the nfs path
will truncated when a user inputs nfs path from kernel cmdline

enlarge nfs_root_parms to 1024, to make it accept the 1024 bytes long
directory name, since nfs_root_parms is defined as _initdata, it will
be released after system bootup

Signed-off-by: Li RongQing <roy.qing.li@gmail.com>
Signed-off-by: Trond Myklebust <trond.myklebust@primarydata.com>