9p: p9dirent_read: check network-provided name length

9p: p9dirent_read: check network-provided name length

[ Upstream commit ef5305f1f72eb1cfcda25c382bb0368509c0385b ]

strcpy to dirent->d_name could overflow the buffer, use strscpy to check
the provided string length and error out if the size was too big.

While we are here, make the function return an error when the pdu
parsing failed, instead of returning the pdu offset as if it had been a
success...

Link: http://lkml.kernel.org/r/1536339057-21974-4-git-send-email-asmadeus@codewreck.org
Addresses-Coverity-ID: 139133 ("Copy into fixed size buffer")
Signed-off-by: Dominique Martinet <dominique.martinet@cea.fr>
Signed-off-by: Sasha Levin <sashal@kernel.org>