NFSD: fix nfs4_file access extra count in nfsd4_add_rdaccess_to_wrdeleg

In the Linux kernel, the following vulnerability has been resolved:

NFSD: fix nfs4_file access extra count in nfsd4_add_rdaccess_to_wrdeleg

In nfsd4_add_rdaccess_to_wrdeleg, if fp->fi_fds[O_RDONLY] is already set by another thread, __nfs4_file_get_access should not be called to increment the nfs4_file access count since that was already done by the thread that added READ access to the file. The extra fi_access count in nfs4_file can prevent the corresponding nfsd_file from being freed.

When stopping nfs-server service, these extra access counts trigger a BUG in kmem_cache_destroy() that shows nfsd_file object remaining on __kmem_cache_shutdown.

This problem can be reproduced by running the Git project's test suite over NFS.