tty/vt: use guard()s in con_font_set/get() and con_{set,get}_unimap()

tty/vt: use guard()s in con_font_set/get() and con_{set,get}_unimap()

Having all the new guards, use them in the 8250_rsa code. This improves
readability, makes error handling easier, and marks locked portions of
code explicit.

The new __free()-annotated declarations are moved to the allocation points
as is preferred:
https://lore.kernel.org/all/CAHk-=wjvh_LUpa=864joG2JJXs3+viO-kLzLidR2JLyMr4MNwA@mail.gmail.com/

Except font_data in con_font_get(). The scope in there would not match
the expected lifetime. But the declaration is moved closer.

Signed-off-by: "Jiri Slaby (SUSE)" <jirislaby@kernel.org>
Link: https://lore.kernel.org/r/20250814072456.182853-15-jirislaby@kernel.org
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>