vt: keyboard, extract and simplify vt_kdskbsent

Setting of function key strings is now very complex. It uses a global
buffer 'func_buf' which is prefilled in defkeymap.c_shipped. Then there
is also an index table called 'func_table'. So initially, we have
something like this:
char func_buf[] =	"\e[[A\0" // for F1
			"\e[[B\0" // for F2
			...;
char *func_table[] = {
	func_buf + 0, // for F1
	func_buf + 5, // for F2
	... }

When a user changes some specific func string by KDSKBSENT, it is
changed in 'func_buf'. If it is shorter or equal to the current one, it
is handled by a very quick 'strcpy'.

When the user's string is longer, the whole 'func_buf' is reallocated to
allow expansion somewhere in the middle. The buffer before the user's
string is copied, the user's string appended and the rest appended too.
Now, the index table (func_table) needs to be recomputed, of course.
One more complication is the held spinlock -- we have to unlock,
reallocate, lock again and do the whole thing again to be sure noone
raced with us.

In this patch, we chose completely orthogonal approach: when the user's
string is longer than the current one, we simply assign the 'kstrdup'ed
copy to the index table (func_table) and modify func_buf in no way. We
only need to make sure we free the old entries. So we need a bitmap
is_kmalloc and free the old entries (but not the original func_buf
rodata string).

Also note that we do not waste so much space as previous approach. We
only allocate space for single entries which are longer, while before,
the whole buffer was duplicated plus space for the longer string.

Signed-off-by: Jiri Slaby <jslaby@suse.cz>
Link: https://lore.kernel.org/r/20201029113222.32640-12-jslaby@suse.cz
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

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