NFS: Verify symlink inode before caching target

nfs_symlink() copies the symlink target into a folio before issuing the
SYMLINK RPC.  After a successful reply, it caches that folio in the
instantiated inode mapping and assumes that the dentry now names a
symlink.

If the dentry is instantiated with a non-symlink inode, the raw symlink
target folio can be inserted into the wrong mapping.  When that inode is
a directory, reclaim or unmount later calls nfs_readdir_clear_array()
through nfs_dir_aops and interprets the symlink target as a readdir
cache array, which can lead to invalid kfree() calls.

A vmcore from a 4.19-based kernel showed the crash when reclaiming a
directory mapping on unmount:

  Stack trace:
   nfs_readdir_clear_array+0x4d/0x70 [nfs]
   page_cache_free_page.isra.35+0x1a/0x90
   delete_from_page_cache_batch+0x1cf/0x2c0
   truncate_inode_pages_range+0x24d/0x910
   [...]
   nfs_evict_inode+0x15/0x30 [nfs]
   evict+0x115/0x2b0
   dispose_list+0x48/0x60
   evict_inodes+0x16c/0x1b0
   generic_shutdown_super+0x3f/0x120
   nfs_kill_super+0x1b/0x40 [nfs]
   deactivate_locked_super+0x3f/0x70
   cleanup_mnt+0x3b/0x80

The current code still has the same unchecked cache insertion pattern,
so it may be susceptible to the same failure mode.

Verify that the instantiated inode is a symlink before caching the
target folio.  If the type is wrong, drop the suspect dentry and skip
the cache insertion while preserving the successful SYMLINK result.

Co-developed-by: Jackie Liu <liuyun01@kylinos.cn>
Signed-off-by: Jackie Liu <liuyun01@kylinos.cn>
Signed-off-by: ZhangGuoDong <zhangguodong@kylinos.cn>
Signed-off-by: Trond Myklebust <trond.myklebust@hammerspace.com>

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