This helps to compare fuse logs of e.g. different test runs. For example
below are two INIT messages with the same flags, but formatted, before
hereby patch, differently:

     rx 2: INIT n0 {7.37 Ra 131072 EXPORT_SUPPORT,SPLICE_READ,AUTO_INVAL_DATA,ASYNC_DIO,WRITEBACK_CACHE,BIG_WRITES,NO_OPEN_SUPPORT,PARALLEL_DIROPS,POSIX_ACL,EXPLICIT_INVAL_DATA,NO_OPENDIR_SUPPORT,ASYNC_READ,DONT_MASK,SPLICE_MOVE,IOCTL_DIR,READDIRPLUS,READDIRPLUS_AUTO,HANDLE_KILLPRIV,POSIX_LOCKS,ATOMIC_O_TRUNC,SPLICE_WRITE,FLOCK_LOCKS,ABORT_ERROR,MAX_PAGES,CACHE_SYMLINKS,0x70000000} "\x03\x00\x00\x00\x00\x00\x00\x00"... 48b

     rx 2: INIT n0 {7.37 Ra 131072 EXPORT_SUPPORT,SPLICE_READ,AUTO_INVAL_DATA,ASYNC_DIO,WRITEBACK_CACHE,BIG_WRITES,NO_OPEN_SUPPORT,PARALLEL_DIROPS,POSIX_ACL,EXPLICIT_INVAL_DATA,ASYNC_READ,DONT_MASK,SPLICE_MOVE,IOCTL_DIR,READDIRPLUS,READDIRPLUS_AUTO,HANDLE_KILLPRIV,NO_OPENDIR_SUPPORT,POSIX_LOCKS,ATOMIC_O_TRUNC,SPLICE_WRITE,FLOCK_LOCKS,ABORT_ERROR,MAX_PAGES,CACHE_SYMLINKS,0x70000000} "\x03\x00\x00\x00\x00\x00\x00\x00"... 48b

which leads to corresponding lines to be present in the diff and
requires human attention to verify that the messages are really the
same.

After this patch above INIT always formats as

     rx 2: INIT n0 {7.37 Ra 131072 ASYNC_READ,POSIX_LOCKS,ATOMIC_O_TRUNC,EXPORT_SUPPORT,BIG_WRITES,DONT_MASK,SPLICE_WRITE,SPLICE_MOVE,SPLICE_READ,FLOCK_LOCKS,IOCTL_DIR,AUTO_INVAL_DATA,READDIRPLUS,READDIRPLUS_AUTO,ASYNC_DIO,WRITEBACK_CACHE,NO_OPEN_SUPPORT,PARALLEL_DIROPS,HANDLE_KILLPRIV,POSIX_ACL,ABORT_ERROR,MAX_PAGES,CACHE_SYMLINKS,NO_OPENDIR_SUPPORT,EXPLICIT_INVAL_DATA,0x70000000} "\x03\x00\x00\x00\x00\x00\x00\x00"... 48b

and the diff log does not unnecessarily distract attention to it.

flagString is used for many messages, this fix should benefit all of
them.

Change-Id: Id4bdab9dd90552313ede41a3b9a6c337588e4b3e