NFS: Add fs_context support.

Add filesystem context support to NFS, parsing the options in advance and attaching the information to struct nfs_fs_context. The highlights are: (*) Merge nfs_mount_info and nfs_clone_mount into nfs_fs_context. This structure represents NFS's superblock config. (*) Make use of the VFS's parsing support to split comma-separated lists (*) Pin the NFS protocol module in the nfs_fs_context. (*) Attach supplementary error information to fs_context. This has the downside that these strings must be static and can't be formatted. (*) Remove the auxiliary file_system_type structs since the information necessary can be conveyed in the nfs_fs_context struct instead. (*) Root mounts are made by duplicating the config for the requested mount so as to have the same parameters. Submounts pick up their parameters from the parent superblock. [AV -- retrans is u32, not string] [SM -- Renamed cfg to ctx in a few functions in an earlier patch] [SM -- Moved fs_context mount option parsing to an earlier patch] [SM -- Moved fs_context error logging to a later patch] [SM -- Fixed printks in nfs4_try_get_tree() and nfs4_get_referral_tree()] [SM -- Added is_remount_fc() helper] [SM -- Deferred some refactoring to a later patch] [SM -- Fixed referral mounts, which were broken in the original patch] [SM -- Fixed leak of nfs_fattr when fs_context is freed]

DHDavid Howells committed 6 years ago