The return value check of the write call is completely bogus. We
check if we have written at minimum sizeof(struct envfs_super) bytes
instead of all bytes. Properly check for all bytes written instead
and allow write to write less bytes than requested.
Do not use write_full because this file is compiled for userspace
aswell.

Signed-off-by: Sascha Hauer <s.hauer@pengutronix.de>