Move RWF_* constants into linux/gnu/mod.rs

From `linux/mod.rs`.  These constants are not exposed by musl so were
causing failures in CI.

These constants are really defined in `include/uapi/linux/fs.h` in Linux
and are not specific to any libc and I hope to make them more available in
the future.

Author: wmanley

src/unix/linux_like/linux/gnu/mod.rs

@@ -860,6 +860,15 @@ pub const EPOLLWAKEUP: ::c_int = 0x20000000;
 pub const SEEK_DATA: ::c_int = 3;
 pub const SEEK_HOLE: ::c_int = 4;
 
+// linux/fs.h
+
+// Flags for preadv2/pwritev2
+pub const RWF_HIPRI: ::c_int = 0x00000001;
+pub const RWF_DSYNC: ::c_int = 0x00000002;
+pub const RWF_SYNC: ::c_int = 0x00000004;
+pub const RWF_NOWAIT: ::c_int = 0x00000008;
+pub const RWF_APPEND: ::c_int = 0x00000010;
+
 // linux/rtnetlink.h
 pub const TCA_PAD: ::c_ushort = 9;
 pub const TCA_DUMP_INVISIBLE: ::c_ushort = 10;

src/unix/linux_like/linux/mod.rs

@@ -1440,13 +1440,6 @@ pub const SYNC_FILE_RANGE_WAIT_BEFORE: ::c_uint = 1;
 pub const SYNC_FILE_RANGE_WRITE: ::c_uint = 2;
 pub const SYNC_FILE_RANGE_WAIT_AFTER: ::c_uint = 4;
 
-// Flags for preadv2/pwritev2
-pub const RWF_HIPRI: ::c_int = 0x00000001;
-pub const RWF_DSYNC: ::c_int = 0x00000002;
-pub const RWF_SYNC: ::c_int = 0x00000004;
-pub const RWF_NOWAIT: ::c_int = 0x00000008;
-pub const RWF_APPEND: ::c_int = 0x00000010;
-
 pub const AIO_CANCELED: ::c_int = 0;
 pub const AIO_NOTCANCELED: ::c_int = 1;
 pub const AIO_ALLDONE: ::c_int = 2;