Fix for smb4php fseek

The stream_fseek function must return a boolean which will be mapped
to 0 for success and -1 for failure for the caller.

This patch fixes stream_fseek of smb4php to also respect this
convention.

Since the encryption app is relying on fseek to detect whether a file is
encrypted by reading the last bit of data, this will fix #5023
Merge pull request #5889 from owncloud/extstorage-smb-brokenfseek

Fix for smb4php fseek