common, os: Perform xattr handling based on detected fs type

In FileStore::_detect_fs() store discovered filesystem type in m_fs_type
Add per-filesystem filestore_max_inline_xattr_size_* variants
Add per-filesystem filestore_max_inline_xattrs_* variants
New function set_xattr_limits_via_conf()
  Set m_filestore_max_inline_xattr_size based on override or fs type
  Set m_filestore_max_inline_xattrs based on override or fs type
Handle conf change of any relevant value by calling set_xattr_limits_via_conf()
Change filestore_max_inline_xattr_size to override if non-zero
Change filestore_max_inline_xattrs to override if non-zero

Fixes: #6143

Signed-off-by: David Zafman <david.zafman@inktank.com>