nfsd: add a nfserrno mapping for -E2BIG to nfserr_fbig

nfsd: add a nfserrno mapping for -E2BIG to nfserr_fbig

I saw this pop up with some pynfs testing:

    [  123.609992] nfsd: non-standard errno: -7

...and -7 is -E2BIG. I think what happened is that XFS returned -E2BIG
due to some xattr operations with the ACL10 pynfs TEST (I guess it has
limited xattr size?).

Add a better mapping for that error since it's possible that we'll need
it. How about we convert it to NFSERR_FBIG? As Bruce points out, they
both have "BIG" in the name so it must be good.

Also, turn the printk in this function into a WARN() so that we can get
a bit more information about situations that don't have proper mappings.

Signed-off-by: Jeff Layton <jlayton@primarydata.com>
Signed-off-by: J. Bruce Fields <bfields@redhat.com>