bcachefs: move bch2_xattr_handlers to .rodata

bcachefs: move bch2_xattr_handlers to .rodata

A series posted previously moved all of the `struct xattr_handler`
tables to .rodata for each filesystem [1].

However, this appears to have been done shortly before bcachefs was
merged, so bcachefs was missed at that time.

Link: https://lkml.kernel.org/r/20230930050033.41174-1-wedsonaf@gmail.com
Cc: Wedson Almeida Filho <wedsonaf@gmail.com>
Signed-off-by: Thomas Bertschinger <tahbertschinger@gmail.com>
Signed-off-by: Kent Overstreet <kent.overstreet@linux.dev>