fs: move timespec validation into utimes_common

Consolidate the validation of the timespec from the two callers into
utimes_common.  That means it is done a little later (e.g. after the
path lookup), but I can't find anything that requires a specific
order of processing the errors.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Acked-by: Linus Torvalds <torvalds@linux-foundation.org>