efi_loader: Avoid underflow when calculating remaining var store size

The efi_var_mem_free() function calculates the available size for a new
EFI variable by subtracting the occupied buffer size and the overhead
for a new variable from the maximum buffer size set in Kconfig. This
is then returned as QueryVariableInfo()'s RemainingVariableStorageSize
output.

This can underflow as the calculation is done in and processed as
unsigned integer types. Check for underflow before doing the subtraction
and return zero if there's no space.

Fixes: f1f990a8 ("efi_loader: memory buffer for variables")
Signed-off-by: Alper Nebi Yasak <alpernebiyasak@gmail.com>
Reviewed-by: Heinrich Schuchardt <heinrich.schuchardt@canonical.com>