NFSD: Remove open coding of string copy

Instead of open coding a __dynamic_array(), use the __string() and
__assign_str() helper macros that exist for this kind of use case.

Part of an effort to remove deprecated strlcpy() [1] completely from the
kernel[2].

[1] https://www.kernel.org/doc/html/latest/process/deprecated.html#strlcpy
[2] https://github.com/KSPP/linux/issues/89

Fixes: 3c92fba5

 ("NFSD: Enhance the nfsd_cb_setup tracepoint")
Signed-off-by: Azeem Shaikh <azeemshaikh38@gmail.com>
Reviewed-by: Jeff Layton <jlayton@kernel.org>
Signed-off-by: Chuck Lever <chuck.lever@oracle.com>