fstests: check for filesystem FS_IOC_FSSETXATTR support

fstests: check for filesystem FS_IOC_FSSETXATTR support

With "_require_xfs_io_command chattr <letter>", check that
flag can be set/cleared using FS_IOC_FSSETXATTR ioctl, similar
to "_require_chattr <letter>" and FS_IOC_SETFLAGS ioctl.

Update the documentation and the tests that use
"_require_xfs_io_command chattr" to test filesystem support
and not only xfs_io support.

Signed-off-by: Amir Goldstein <amir73il@gmail.com>
Reviewed-by: Eryu Guan <guaneryu@gmail.com>
Signed-off-by: Eryu Guan <guaneryu@gmail.com>