nfs: Mark unnecessarily extern functions as static

nfs_initialise_sb() and nfs_clone_super() are declared as extern even
though they are used only in fs/nfs/super.c. Mark them as static.

Also remove explicit 'inline' directive from nfs_initialise_sb() and
leave it upto compiler to decide whether inlining is worth it.

Signed-off-by: Jan Kara <jack@suse.cz>
Signed-off-by: Trond Myklebust <trond.myklebust@primarydata.com>