tmpfs: Fix OOB write when setting vfs.tmpfs.memory_percent

tmpfs_mem_percent is an int not a long, so on a 64-bit system this
writes 4 bytes past the end of the variable. The read above is correct,
so this was likely a copy paste error from sysctl_mem_reserved.

Found by: CHERI
Fixes: 636592343c3e ("tmpfs: increase memory reserve to a percent of available memory + swap")