Bug in fcntl and fcntl64 implementations

[emkaix]

Describe the bug

https://github.com/qilingframework/qiling/blob/dev/qiling/os/posix/syscall/fcntl.py#L176
The current implementations of fcntl and fcntl64 on the dev branch don't return the "lowest numbered available file descriptor greater than or equal to arg", as described in the man pages https://linux.die.net/man/2/fcntl64. arg is used as the second argument for enumerate, which only affects idx, but not val https://docs.python.org/3/library/functions.html#enumerate. Because of this, idx returned by enumerate doesn't correspond to the real index in ql.os.fd for a given val, in case arg is greater than 0.

Example Scenario:

File descriptor 0: <in use>
File descriptor 1: None
File descriptor 2: <in use>
File descriptor 3: <in use>

fcntl64(fd = 0x3, cmd = F_DUPFD, arg = 0x1) should return 1, but instead 2 is returned, which breaks functionality.

Expected behavior

fcntl64 and fcntl syscalls with cmd==F_DUPFD should return the "lowest numbered available file descriptor greater than or equal to arg", as described in the man pages https://linux.die.net/man/2/fcntl64

[elicn]

Good catch.
Thanks for reporting, I'll fix it soon.