Auto merge of #57121 - glaubitz:flock-fixes, r=nagisa

flock: Use fcntl constants directly from libc crate on Unix targets

Since the values for the fcntl constants can vary from architecture
to architecture, it is better to use the values defined in the libc
crate instead of assigning literals in the flock code which would
make the assumption that all architectures use the same values.

Fixes #57007

Author: bors