Because we incorrectly reused of variable `i` in `z_erofs_gbuf_exit()`
for inner loop, we may exit early from outer loop resulting in memory
leak. Fix this by using separate variable for iterating through inner loop.
Fixes: f36f3010f676 ("erofs: rename per-CPU buffers to global buffer pool and make it configurable")
Signed-off-by: Sandeep Dhavale <dhavale@xxxxxxxxxx>