nfsd: Mark variable __maybe_unused to avoid W=1 build break

nfsd: Mark variable __maybe_unused to avoid W=1 build break

Clang is not happy about set but (in some cases) unused variable:

fs/nfsd/export.c:1027:17: error: variable 'inode' set but not used [-Werror,-Wunused-but-set-variable]

since it's used as a parameter to dprintk() which might be configured
a no-op. To avoid uglifying code with the specific ifdeffery just mark
the variable __maybe_unused.

The commit [1], which introduced this behaviour, is quite old and hence
the Fixes tag points to the first of the Git era.

Link: https://git.kernel.org/pub/scm/linux/kernel/git/history/history.git/commit/?id=0431923fb7a1
Fixes: 1da177e4c3f4 ("Linux-2.6.12-rc2")
Cc: stable@vger.kernel.org
Signed-off-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Signed-off-by: Chuck Lever <chuck.lever@oracle.com>