storage: fix out of bounds read

Given that shadow_len is size_t (unsigned integer), subtracting a number
from it will make it wrap around < 0 and become positive again so the
subsequent "if (n > 0)" check will be mostly useless. On AOSP this makes
rmtfs segfault, on Linux distributions rmtfs happily reads beyond the
end of the buf.

Fix this by casting both parameters to ssize_t (which is signed) to
correctly use the if and not read beyond the end of shadow_buf.

Relevant trace using extra debug statements:
  storage_populate_shadow_buf: file=/dev/disk/by-partlabel/fsg shadow_buf=0xffffa5217060 shadow_len=0x280000
  <snip>
  storage_pread: memcpy shadow_buf=0xffffa5217060 offset=0x27fc00 n=0x200
  storage_pread: memcpy shadow_buf=0xffffa5217060 offset=0x27fe00 n=0x200
  storage_pread: memcpy shadow_buf=0xffffa5217060 offset=0x280000 n=0x0 - don't read!
  storage_pread: memcpy shadow_buf=0xffffa5217060 offset=0x280200 n=0x200
  storage_pread: memcpy shadow_buf=0xffffa5217060 offset=0x280400 n=0x200
  storage_pread: memcpy shadow_buf=0xffffa5217060 offset=0x280600 n=0x200
  storage_pread: memcpy shadow_buf=0xffffa5217060 offset=0x280800 n=0x200
  <snip>

Signed-off-by: Luca Weiss <luca.weiss@fairphone.com>