Claude review: fbdev: Wrap user-invoked calls to fb_set_var() in helper

[Claude Code Review Bot <claude-review@example.com>]

Patch Review

**Correctness**: The new `fb_set_var_from_user()` correctly consolidates the three-step pattern: check with `fbcon_modechange_possible()`, call `fb_set_var()`, then `fbcon_update_vcs()`. This is a clean factoring.

**Behavioral fix for sysfs path**: The `activate()` function in `fbsysfs.c` previously skipped `fbcon_modechange_possible()`. Now it calls it via the helper. This is a fix for an existing inconsistency, as the commit message notes — good.

**Behavioral change in sh_mobile_lcdcfb**: The original code at `sh_mobile_lcdcfb.c:1775`:
```c
var.activate = FB_ACTIVATE_NOW;
...
fbcon_update_vcs(info, true);
```
becomes (via `fb_set_var_from_user`):
```c
fbcon_update_vcs(info, var->activate & FB_ACTIVATE_ALL);
```
Since `FB_ACTIVATE_NOW` is 0 and `FB_ACTIVATE_ALL` is 64, the `all` parameter changes from `true` to `false`. This means only the current VC gets updated rather than all VCs. This may be the intended fix (the cover letter says "fix inconsistencies among the various code paths"), but it's a user-visible behavioral change that should be explicitly mentioned in the commit message.

**EXPORT_SYMBOL**: `fb_set_var_from_user` is exported with `EXPORT_SYMBOL` (not `_GPL`), matching `fb_set_var`. Fine since it's used by `ps3fb` and `sh_mobile_lcdcfb` which can be modules.

**Minor**: The removed `EXPORT_SYMBOL(fbcon_update_vcs)` and `EXPORT_SYMBOL_GPL(fbcon_modechange_possible)` in fbcon.c are now unnecessary since only fbmem.c calls them — this is correct.

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