xfs: convert buffer flags to unsigned.

5.18 w/ std=gnu11 compiled with gcc-5 wants flags stored in unsigned
fields to be unsigned. This manifests as a compiler error such as:

/kisskb/src/fs/xfs/./xfs_trace.h:432:2: note: in expansion of macro 'TP_printk'
  TP_printk("dev %d:%d daddr 0x%llx bbcount 0x%x hold %d pincount %d "
  ^
/kisskb/src/fs/xfs/./xfs_trace.h:440:5: note: in expansion of macro '__print_flags'
     __print_flags(__entry->flags, "|", XFS_BUF_FLAGS),
     ^
/kisskb/src/fs/xfs/xfs_buf.h:67:4: note: in expansion of macro 'XBF_UNMAPPED'
  { XBF_UNMAPPED,  "UNMAPPED" }
    ^
/kisskb/src/fs/xfs/./xfs_trace.h:440:40: note: in expansion of macro 'XFS_BUF_FLAGS'
     __print_flags(__entry->flags, "|", XFS_BUF_FLAGS),
                                        ^
/kisskb/src/fs/xfs/./xfs_trace.h: In function 'trace_raw_output_xfs_buf_flags_class':
/kisskb/src/fs/xfs/xfs_buf.h:46:23: error: initializer element is not constant
 #define XBF_UNMAPPED  (1 << 31)/* do not map the buffer */

as __print_flags assigns XFS_BUF_FLAGS to a structure that uses an
unsigned long for the flag. Since this results in the value of
XBF_UNMAPPED causing a signed integer overflow, the result is
technically undefined behavior, which gcc-5 does not accept as an
integer constant.

This is based on a patch from Arnd Bergman <arnd@arndb.de>.

Reported-by: Geert Uytterhoeven <geert@linux-m68k.org>
Signed-off-by: Dave Chinner <dchinner@redhat.com>
Reviewed-by: Chandan Babu R <chandan.babu@oracle.com>
Signed-off-by: Dave Chinner <david@fromorbit.com>