nsfs: unobfuscate

1) IS_ERR(p) && PTR_ERR(p) == -E... is spelled p == ERR_PTR(-E...)
2) yes, you can open-code do-while and sometimes there's even
a good reason to do so.  Not in this case, though.

Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>