ext4 crypto: Use a larger on-stack file name buffer

32 bytes for the on-stack file name buffer is rather small and doesn't
fit many file names, causing dynamic allocation to be used more often
than necessary instead. Increasing the on-stack buffer to 1 KiB is safe
and helps this function avoid dynamic memory allocations far more
frequently.

Signed-off-by: Sultan Alsawaf <sultan@kerneltoast.com>