NFSv4.2: Move TRACE_DEFINE_ENUM(NFS4_CONTENT_*) under CONFIG_NFS_V4_2

NFS4_CONTENT_DATA and NFS4_CONTENT_HOLE both only exist under NFS v4.2.
Move their corresponding TRACE_DEFINE_ENUM calls under this Kconfig
option.

Signed-off-by: Anna Schumaker <Anna.Schumaker@Netapp.com>