Loop on `EINTR` on file lock acquisition

[zanieb]

We should loop on EINTR, and probably in the blocking cases too, unless I'm missing something that already does that.

Originally posted by @geofft in #15992 (comment)

[zanieb]

I'm not sure if something below us takes care of this.

[aznszn]

I was looking into this and it seems like nothing below UV takes care of this

in fs_err

fn try_lock_exclusive(&self) -> Result<()> {
    sys::try_lock_exclusive(self)
}

pub fn try_lock_exclusive(file: &File) -> Result<()> {
    flock(file, libc::LOCK_EX | libc::LOCK_NB)
}

#[cfg(not(target_os = "solaris"))]
fn flock(file: &File, flag: libc::c_int) -> Result<()> {
    let ret = unsafe { libc::flock(file.as_raw_fd(), flag) };
    if ret < 0 { Err(Error::last_os_error()) } else { Ok(()) }
}

so should we unconditionally loop on EINTR until we get the lock or an error other than EINTR, or should we define something like MAX_EINTR_RETRIES after which we return a custom too many retries error