selinux: improve error checking in sel_write_load()

[ Upstream commit 42c773238037c90b3302bf37a57ae3b5c3f6004a ]

Move our existing input sanity checking to the top of sel_write_load()
and add a check to ensure the buffer size is non-zero.

Move a local variable initialization from the declaration to before it
is used.

Minor style adjustments.

Reported-by: Sam Sun <samsun1006219@gmail.com>
Signed-off-by: Paul Moore <paul@paul-moore.com>
[cascardo: keep fsi initialization at its declaration point as it is used earlier]
[cascardo: keep check for 64MiB size limit]
Signed-off-by: Thadeu Lima de Souza Cascardo <cascardo@igalia.com>
Signed-off-by: Sasha Levin <sashal@kernel.org>